Braking issues on 74 w disc front

[Outta_OC]

Good morning ratsun.net,

 

I've been having brake issues the last few days: pedal fade prettyuch to the floor, increased idle speed when I press on the brake, and a "leaking air" sound when I press on the brakes.

 

I have locked up the brakes before, however in their current state, there is not enough pressure to get them to lock up.

 

When the truck is off, the pedal feels normal and depresses about half way before stopping.

 

Any suggestions? I got under the truck and checked my brake lines, I have fluid, could I have a hole in the bladder of my master cylinder?

[hobospyder]

Vacuum leak in your brake booster?

 

And yeah, check all the seals and diaphragms in the master.

[thisismatt]

Vacuum leak in the hose to the booster, or in the booster itself.  That said, I didn't think 74 620's had disc brakes, so you might want to post pics or more info about the brake system otherwise you might end up with the wrong parts or not be able to find the parts.

[Outta_OC]

So... If I have the e-brake engaged and step on the brake pedal, there is NO "leaking air" sound and no increase in RPM.

 

Wtf?

[thisismatt]

ebrake linkage/cable hitting the brake booster hose?  It might just be coincidental. 

[Outta_OC]

Perhaps coincidence... Exterminator just showed up gotta take a break

[Outta_OC]

 

I doubt this will help, but as I was pulling my tire off to get a pic of the front brakes the exterminator showed up... Bbiab

[banzai510(hainz)]

when you pull the e brake your taking the extra slop out of the system.

Maybe just adjust the rear brakes 1st

 

 

never had a boosted brakes on Datsun so Im just winging it

[Outta_OC]

Got put on hold until tomorrow; I got called into work :/

[datzenmike]

A leak I the hose to the booster would leak all the time.

 

The leaking air sounds is likely vacuum being bled from the booster when brakes applied. There is a soft felt? diffuser on the vacuum booster right where the push rod enters it from the cab just above your feet just for the purpose of reducing this noise. It may have fallen out.

 

 

 

To test the brake booster action do the following...

 

Pump brakes several times to remove any residual vacuum stored.

Press down on the brake and hold.

Start engine.

You should notice the brake pedal drop slightly, maybe an inch as engine intake vacuum rises.

 

If so, the vacuum assist is functioning.

 

I'm with Hainz. Badly out of adjustment rear drums eat up huge amounts of pedal travel. Loosen the emergency brake cable. Get the rear drums adjusted, then adjust the emergency brakes. This will likely solve your brake problems.

[Outta_OC]

Welp, pulled the back tires and adjusted the rear brakes, it did make a difference in how far I have to pull the e-brake but it did not fix the original problem or improve pedal feel. So I ordered a new power brake booster that will be avail. For pickup tomorrow morning.

 

Thanks for all the tips and troubleshooting ratsun.net!

[datzenmike]

Dude if the brake booster is not working the pedal will travel less distance than when it is.  You said it travels too far. A new booster won't change that.

 

Did you loosen the E brake cable first then adjust both rear wheels? Always step on brakes once or twice to center the shoes and then check your adjustment and add more as needed. When fully adjusted set the e brake.

 

 

If you still have soft spongy brakes there may be air trapped in the lines.

 

Do things in order. Eliminate the usual causes of a soft pedal or you'll be buying stuff you don't need. Gets expensive.

 

 

What brake calipers are on the front?

[Outta_OC]

I disengaged the e-brake but I did not physically dislocate the e-brake cable from anything...

[Outta_OC]

I'm not trying to buy unnecessary parts, but I did have a hole in the diaphragm of my 63 galaxie booster once, and I had similar symptoms as I'm having now. Been running around a lot today and haven't been able to pull the master cylinder off and check and see if there is a hole... Tomorrow.

 

Can you perhaps explain why I need to disconnect the e-brake cable? I've adjusted rear drums before on American cars and never disconnected an e-brake cable

[Outta_OC]

As for the fronts, I haven't pulled the tires and checked the calipers, I was just checking for leaks on the back side earlier.

 

And my fluid is full, and there really shouldn't be a reason for any air to be in the lines, there had been nothing done to the brakes to allow air in

[datzenmike]

Dude if the brake booster is not working the pedal will travel less distance than when it is.  You said it travels too far. A new booster won't change that.

 

Did you loosen the E brake cable first then adjust both rear wheels? Always step on brakes once or twice to center the shoes and then check your adjustment and add more as needed. When fully adjusted set the e brake.

 

 

If you still have soft spongy brakes there may be air trapped in the lines.

 

Do things in order. Eliminate the usual causes of a soft pedal or you'll be buying stuff you don't need. Gets expensive.

 

 

What brake calipers are on the front?

 

Actually I said loosen them.

 

Past experience is a double edge sword. It provides experience and insight but at the same time it can cause tunnel vision and the belief that because something else is similar it must be the same.

 

 

I would first adjust the brakes.

Then check for air in the lines.

 

If these do not bring the brakes up to a satisfactory firmness then you've not wasted your time as this needs doing anyway.

 

Disc brake calipers have larger pistons in them and need much more fluid volume from the master to properly work them. If your front brakes are very large, the master may be too small for them. The '78 and up 620 has factory front disc brakes and the master was increased from your 3/4" to 13/16" to provide more fluid volume.

[Outta_OC]

Thanks Mike, I appreciate your insight. I did adjust the back brakes, and after merely taking it off the jacks and not taking it around the block, I was left with the impression that nothing had changed.

 

Well, when i left my old mans house and after driving the truck around the block, the brakes are working better than they ever have since I've owned the truck! And it got me safely to my local bar!

 

Ratsun to the rescue.