just did a brake job and pedal goes down a lot but brakes work

[rjdaaa]

I did a brake job on the 75 stock brake 620,new shoes, wheel cylinders and master cylinder. The master was leaking into the brake booster for a while, so I went through all but did not replace the booster.After I adjusted each brake on each wheel, and bled each wheel, I had a rock hard pedal, but the pedal travels about 3/4 to the floor before there is any resistance. Before I did the job the pedal did not travel more than 1/4 before it stopped. I don't think I have air in the lines as I used a mitty vac and ran a lot of fluid through. I adjusted each wheel so there was just a slight occasional contact with the drum and the wheel still turned. I have driven it aboit 100 miles like this and just ordered a booster, the old booster still works but maybe not as strong as before. I am guessing it is the 25 year old booster but maybe not?

[datzenmike]

Just for the hell of it....

 

Pump brake several times to vent any stored vacuum in the booster. Apply the brake normally and start the engine. As soon as it starts the peddle should drop towards the floor slightly if the booster is working properly. Seem right?

[Aibast]

Mine sank to much when the rear brakes where out of adjustment. Got them tighter and they are ok now. Drum brakes need to be adjusted properly for them to have a good contact. With new bads but old drums you might get some contact but they aren't seated in the old groves yet. So you need to drive them in and readjust.

[rjdaaa]

Datzenmike;

Yes it does not pass your test,pedal drops quite a bit even when I follow the procedure, so maybe when I get the booster and install should take care of it.

Aibast:

Rear brakes are actually adjusted tighter now than when the pedal did not move as much, but you are right, the shoes do settle in after a bit of driving.

thanks for the replies.

rjdaaa

[Llittle_Llama]

Did you not get the drums turned for some reason?

[rjdaaa]

I always checked for vacum leaks when I could not find the source, or wanted to make sure it was a vacum leak, by putting the palm of my hand over the carb throat (with the aircleaner bolt removed) while the engine idled roughly, and slowly moving it closer to the carb throat,  at some point the idle will get higher if there is a vacum leak, as you are cutting off the air through the throat and the  air from the leak is feeding into the engine, since with a vacum leak you are putting too much air into the engine and it is running lean and idling roughly. 

[datzenmike]

my truck is doing some crazy stuff...I cannot get it to Idle right. It idles high but when sitting still and running (idling high) I can hold the brake and slowly let out the clutch so that there is a load on the drivetrain and not enough to kill the engine, and the truck will idle lower without going back higher. With all of the info about other peoples brakes and boosters  I've been reading I seem to have alot of the same issues with mine. Could my booster be the cause of a vacum leak that is making the motor idle high? I've replaced all of the other vacum lines.

 

 

I had intended to fix the rough idle before moving on to the brakes, but whatever gets it goin'

 

 

Perhaps it would be more polite to start your own post about your problems than jump into the middle of someone else's and interrupt it. ????

[lostforawhile]

did you bench bleed the master? you have to because when it's on the booster, there isnt enough pushrod travel to get all the air out of it, you have to get the kit with the plastic fittings that screw into the ports, you run plastic tubes into the reservoir, fill it with brake fluid, then use something to push the piston in the cylinder until all the air bubbles are gone, any time the master is empty it has to be bench bled again off the car

[lostforawhile]

here is a how to, use the method of the tubes into the reservoir, it's much easier, you can get a kit with the fittings at most auto parts stores    http://autorepair.about.com/od/fixityourself/ss/MC_bench-bleed.htm

[rjdaaa]

Yes I did bench bleed the master, ran the tubes into the 2 filled reservoirs and pumped all the way till I had fluid only moving. Also, I mity vaced each wheel and moved a lot of fluid-refilled reservoirs a lot. If the new booster doesn't cure the problem I will look at air in the master again, however usually air means mushy pedal, the pedal is rock hard once it moves 3/4 of it's travel. Still driving it and booster seems like it may not be boosting as much, as it used to. I will report back once I get the new booster on.

Thanks for the replies.

[Llittle_Llama]

Be safe until then though :)

[Cardinal Grammeter]

I have the same problem:  pedal goes down 1/2 stroke or more and solid brakes

 

EDIT: 

 

To save you the both of reading a dozen or so posts, I'll give you the Spoiler:  (lots of info in those posts if you care to read them)

 

SOLVED:  Booster to MC pushrod was massively too short. 

 

I'm going to "hijack" this thread because while it is abandoned, I have the exact same problem - I'm not going to start a new thread.

 

NOTE:  I've also PM'd the OP to see what happened when he got his new booster - perhaps it worked and he never posted back?

 

So here are my details:

 

 

 

Truck easily sat 5-10 years since previous owner(s) put brakes on it, so keep that in mind.

 

1.5 years ago when I bot it:

• I was told all it needed was a master cylinder which was included, NIB.
• I'm pretty sure I bled it because I seem to recall having to rig up the tubing loops to do it.
• Inspected all 4 brakes and checked for leaky wheel cylinders - found none.
• Bled system and had good pedal with no issues

1 month ago:  (ready to drive, better check brakes again)

• No brakes - pedal goes to floor (that was original impression, may have actually be going 3/4 the way and seemed like going to floor - which is probably true, because I thought something blew out, pumped and pumped, nothing, and no fluid loss.)
• Bled system again, this time got lots of brown mud out of the system which is a mystery because I swear I did it 1 year ago
• Stuck wheel cylinders in both rears.  Freed them up by pushing them in with a steel ball and C-clamp.  TRICK:  Used a C-clamped small freeze plug cup type to limit their protrusion when cycling them.

This left me with the following symptoms:

• Brake pedal goes down 1/2 its stroke before anything happens.
• At 1/2 stroke, pedal goes nice and solid and good brakes
• Brakes do not pump up (it's possible they do but only a tiny amount)

 

Current state of the system:

• Shoes are 100%, in fact, it is extremely difficult to remove the rear drums with the adjusters backed all the way out.
• Fronts adjusted until "they just started to get tight" minus one click on the fronts
• Bleed many times
• Master cylinder bled?  I think so, but I would have done it 1 year ago and frankly I'm not sure)
• Booster:  did the test:  engine off, apply breaks hard and hold, start engine, feel pedal go down.  Yes it goes down but just barely - not like the big movement in other cars I've done this with.

Theories:

• NOT brake adjustment:  they are all way far tighter than the "back off 12 clicks" in the FSM.
• NOT air in system:  pedal is solid and does not pump up. 
• Master Cylinder no bled:  Could this be possible with solid pedal that does not pump up?
• Power Brake Unit:  Does not appear to have vacuum leak since when brakes applied, engine speed does not change.
• Free Play:  When pedal is pressed, it moves a small amount 1/4-1/2" and you can feel some rod touch something else.  While not a solution, something to be adjusted?

[datzenmike]

Free play should just be a few mm or 1/16". Just the bare minimum to know that there is play.

 

What size diameter was the new in the box master you put on? I imagine it was a cheap re-build?

[Cardinal Grammeter]

It's marked "3/4" and has "JAPAN" on it too.

 

It was in the "remains" of a box which I tossed long ago.  There were a few specs of oxidation on the casting body as if it sat probably in the interior of the truck while it (the truck) was corroding away under a tree (cowl filled with 2 "bricks" of tree "droppings.")

 

The Free Play sounds reasonable but could be reduced a little since way more than enough for me to detect.

 

If its the MC, I just don't see the failure mode to cause all that travel.

 

Oh, I forgot two basic questions:

1. What is the best "pedal stroke to solid" I can expect?
2. Like most old trucks, the rears lock first since designed to work at full load - is 74 like this?

Oh, one observation of why brakes are not loose:  If drum brakes are not adjusted up and "loose," when you apply brakes, they don't all engage at once so there is usually a momentary pull until they all pressurize.  These brakes are straight as an arrow - except for the one rear that is grabby when cold.

 

Also, the emergency brake works perfectly.

[datzenmike]

Causes of excess pedal travel...

 

Brake shoes need adjusting closer to the drum

Too much pedal free play.

Too small a master.

Failed residual valve or disc brake master cylinder installed.

 

 

1/2 the pedal travel is fine as long as pedal is then firm and the brakes work well. Your leg strength increases as the leg extends. (ask anyone that rides a bike) So you don't want it too firm up at the top of the travel. When your leg is more extended it's much easier to add or remove braking to control locked up tires.

[DanielC]

On a 521. there is only one port on the slave cylinders.  I think 620 slave cylinders are the same way.

 

On cars with two ports on the slave cylinders, vacuum bleeding sucks fluid through the brake line port, through the slave cylinder, and out the bleeder port, along with any air in the slave cylinder.  With a single port slave cylinder, you only bleed air in the brake lines, between the bleeder valve, and master cylinder, and not any air in the slave cylinder.

 

Try bleeding the brakes the old fashioned way, two people, one stepping on the brake pedal, and the other person opening and closing the bleeder valve.  With the brake pedal held down, open the bleeder fitting, and wait until fluid flow stops, then close the bleeder, and after that, let the brake pedal up, and hold down on the brake pedal again.

You need to allow the brake shoe springs to collapse the slave cylinder pistons, to push air back out the bleeder fitting.

[Cardinal Grammeter]

DanielC, 

 

I've always done the 2-man bleed method.  On guy pumps the brakes hard and then pushes down as hard as he can for max pressure,  Then I crack the bleeder, pedal goes to floor, I close the bleeder.

 

But that is where I stop - of course I repeat this process until the fluid is clear AND there are no little "ticks" of air shooting out the bleeder.

 

Your "...and after that, let the brake pedal up, and hold down on the brake pedal again." did you mean repeat the process?

 

But it sounds like solid pedal at 1/2 stroke may be just fine.

 

I was just thinking what is happening when I pump the brakes and the pedal gets a tiny big higher:  This is probably due to the brakes not fully retracting between pumps - so this slightly higher pedal would be due to adjusting the brakes tighter.  So brakes "let go" the instant you take your foot off the pedal, adjusting them a little tighter might be OK, but the rear drums are almost impossible to get off (and there is no ridge in the drums) and the fronts were adjust until the very next adjuster click was definitely going solid.  So there just isn't that much to deal with.

 

It just seems like a lot of pedal stroke with basically no pedal effort.

[yello620]

Most people fail to adjust the Datsun brake system properly. Most people will adjust the till they hear a little drag and then back the off a little. This will result in a low pedal every time, unless you get very lucky.

 

I have to take a couple of assumptions here though. I cannot verify the working order of your brake parts and cylinder sizes. If you have sticking wheel cylinders or wrong size master cylinder, you will always have issues. Along with this is your brake shoes, it will not stop properly if you put the wrong shoes in the wrong locations. Another common issue is the adjusters must be able to slide freely in the backing plate. These things have to be verified by you, providing us with good pictures might help with this though.

 

Do this--De-adjust your parking brake until it doesn't do anything anymore. Adjust all four corners until you fell a good amount of drag/resistance. go pump the brake pedal a couple times. Now go adjust all four corners again, they will be loose again. Might have to do this a couple of times. Do it until they basically will not adjust anymore. Do NOT drive it like this, and it doesn't need to be engine running for any of this. Your low pedal has nothing to do with your booster.

 

Your pedal is starting to feel pretty good right now isn't? Now do a brake bleed on each corner while the brakes a fully adjusted. Now the pedal feels good doesn't it?

 

Right now you have eliminated brake adjustment as a possible issue(assuming you are fully adjusted) in your pedal travel and only fluid/air is left as possibilities. This is assuming that you haven't fucked with things like the booster to master push rod or pedal to booster push rod. If somebody ever adjusted the parking brake cables before/without adjust the rear brakes properly first, it will not allow the rear brakes to be adjusted properly now.

 

Now that you have a good pedal(should barely move). Now you can start de-adjusting the brakes on each corner the 12 clicks the service manual says to do(I usually found that 8-10 clicks was enough though). Finally, now you can re-adjust your parking brake.

 

Now that the brakes are de-adjusted and turning with only a slight drag, you should be good to drive the truck again.

[Cardinal Grammeter]

yellow620, thanks for all the suggestions.  I'll work my way through them:

 

I have no idea if I have the right master cylinder nor do I have any way to determine that.  It came with the truck and bolted on.  It looks proper and has a 3/4" bore.  Considering the weight of the vehicle, a 3/4" bore sounds right.  I've changed master cylinder bores on a number of cars, going smaller for more brake force and stroke has never been an issue.  Even if the wrong cylinder, 3/4" should work.  In fact, I was thinking considering the stroke and manual brake adjusting issue, a larger bore might be a good mod.

 

The adjusters are free and teflon dry lubed.

 

I have no idea if it has the right shoes - I didn't buy them.  The rear shoes are very tight adjusted all the way out and very difficult to get the drums off.  The fronts are not like that.  I didn't accurately measure the shoe thicknesses to determine if the fronts are worn more than the rears which I assume are not worn.  To the naked eye, the fronts are just at thick as the rears.

 

I would assume that with the brakes adjusted properly, it is possible to get the drums off. 

 

The stuck rear cylinders didn't take much to free them.  I cycled them many times until they were quite free.

 

The pedal is solid with no trace of spongy so bleeding is not an issue, however they do pump up a tiny bit which would indicate they could be adjusted tighter.

 

Like you said, you can adjust them tight, but when you apply the brakes, they will be lose again.  The FSM says to tap the adjusters - is it forward? - before you adjust them.  I think I'll tighten up the fronts until it is "barely possible" to get the drums off.

_______________

 

But you know what?  I didn't check the power brake unit pushrod to master cylinder piston clearance.  It is possible there is a ton of clearance there.  And that would feel about right for what I'm feeling in the pedal.  The pedal stroke to solid does not feel like its moving the brakes - that would require some pressure to overcome the springs.  It feels like there is nothing connected until 1/2 stroke is reached.

 

For a test, I think I'll loosen the master cylinder mounting nuts a bunch, push the pedal and see how far it goes before it pushes the MC away from the booster.  Or some other method.

 

Of course, what are the odds that this is the case?  But then, from what I've seen on this truck, the previous owner(s) have been complete morons.

 

But I have a test I'm very excited to perform!

 

Thanks for the suggestions yellow620!

[Rick-rat]

If you do a 4 wheel disc brake set up, you won't need to worry about adjusting brakes again  :thumbup:

[Cardinal Grammeter]

SOLVED:  Booster to MC pushrod was massively too short. 

 

Had to lengthen it almost one quarter inch!  It's a wonder they worked at all.  NICE, solid, high pedal!  Thanks yellow620 for mentioning that - I completely forgot about that adjustment.

 

But it's never easy with this one:

 

I thought of this last night around 2am, and got up a 6:45am to attempt adjustment before my 8:00am inspection appointment.

 

When I get the MC off, it is obvious the PR was way too short.

 

BUT, it was severely rusted!

 

So I pull it out, clamp at the base as much as possible with small visegrips to act as heat sink, and get the oxy-acetylene torch.

 

OUT OF ACETYLENE!  So I have to dig up my spare plumber's bottle and get the adapter.

 

Rounded up some aluminum sheet to keep the flame off the vacuum hoses.

 

And ad to get it red-orange to get it free  (spraying it down with water immediately after torch withdrawn.)

 

Did knock all the tools off the air cleaner and couldn't find the 12mm wrench - it had shot off 6' from where it hit.  It was definitely trying to escape!

 

Anyhow, a very good 45 minutes!  This brake pedal was some serious negative juju.

[datzenmike]

My '73 FSM shows 11/16" master. Later '78 with disc brakes is 13/16".

 

I don't know for sure what the '74 truck is. Maybe 3/4"

[Cardinal Grammeter]

Checked RockAuto and they show 3/4" and mine looks just like the one they show:  (mine even has those wire clamps)  BUT, from that view, mine has a large nearily raise "3/4" between the reservoirs on the side of the casting and a "JAPAN" also on the side just forward of the mounting flange.