replaced master cylinder, no rear brakes.....

[Lee R]

1974 Datsun 620.  Master was leaking so replaced with a new unit, not rebuilt.  Same style as the OEM one and same fit but noe cannot bleed rears and pedal is to the floor.

 

1. Bench bled the master while installed (plugged brake lines and used the bleeders on the side), no air, perfect firm pedal. 

2. Take out the brake line plugs and connect the brake lines...pedal goes to floor.

3.  attach bleeder line and bottle to passenger rear, open bleeder and it dribbles out.

4.  pump brakes 5 times and very little comes out.

5.  pump some more and nothing.  this is the same on both passenger rear and drivers rear.

 

fronts are fine....no leaks in lines or cylinders.  Fronts are disc and the calipers are not leaking either.

 

Bummed.......

[wayno]

The issue is likely you bled the front brakes first, try putting a piece of aquarium air hose on one of the front caliper bleed screws, and put the other end in a cup of brake fluid, then open the screw a little.

Now bleed the back brakes while making sure both master brake reservoirs do not run dry.

You start bleeding brakes the farthest away from the master and work your way to it.

Once the backs are bled, then close the front screw and remove the tubing, you should be done now unless you have another issue.

[Lee R]

^ nope....started at the passenger rear.  Didn't even get to bleeding the fronts.  All I did was have someone push the brake as I was turning the front rotors and they stopped.  If I do the same on the rear drum, I can still rotate the rears with slight resistance.  Also, if bleeding with two people, I will open the rear bleeder screw as the pedal is depressed and I get a "pfft" of air followed by a dribble of fluid then I close it off.  When I do it again, it is less air and less fluid.  Fluid stays full....might go down 1/8 an inch or even less!

 

I'll try what you said about the fronts and see if it works.  thanks!

[Eagle_Adam]

NM - deleted 

[K_trip]

- removed -

[DanielC]

1.Get your helper in the car, and you under passenger rear.

2.Have your helper rapidly pump the brake pedal many times until the brake pedal is firm.

3.When the pedal is firm, and the helper is holding pressure, you open the bleed screw.  until the pedal goes to the floor.

4.DO NOT HAVE YOUR HELPER LET THE PEDAL UP UNTIL YOU CLOSE THE BRAKE BLEED FITTING!

5.After you close the bleed fitting, then your helper can pump the brakes up again.

6.Repeat steps 2 through 5 twice, and check the fluid level in the master cylinder.  Add fluid.

7.If you are getting any air, keep bleeding the passenger rear until no air. 

8. If no air, then you can move to the other rear wheel, and repeat steps 2 through 7.

[banzai510(hainz)]

I would have the helper pump the pedal while you have the bleed open in a clear long tube till fluid come out in plastic bottle (keep filling the resivoir). then it looks preety good go to the next rear ,same proecess. lock it when you see no more bubbles.

 

now push on the brake and if you see airbubble pop up  and still hits the floor the master is bad.

 

I assume your Master is adjusted correctly thru the fire wall as every one I have done the RODS are too long pushing on the pedal thus pusing the rod into the Master cylinder and need to be cut(threaded rod) and sized for full movement and release.

[Lee R]

Truck is level and I know the bleeding procedure.  Done it many times before on different vehicles but this is the first where there is just oozing fluid from the rear.  Master is brand new, not rebuilt.  It's possible it it could be bad, but i think that chance is slim. 

 

The fluid just oozes from the rear no matter how much pumping.  It will only enter the clear tube up to an inch and never make it to the bottle. 

 

The only thing I have not done is the adjustment of the rod in the firewall.  Would this adjusment change or need to be made when just swapping out the old MC with a new one?  Could a vacumm leak in the booster be a problem?  It seems fine to me.

[ggzilla]

Lately there's been a bunch of bad MC right out of the box

[datzenmike]

 Could a vacumm leak in the booster be a problem?  It seems fine to me.

 

Booster has no effect on the master when engine off. Only reduces effort when engine on. 

 

If air is coming out.

 

4.DO NOT HAVE YOUR HELPER LET THE PEDAL UP UNTIL YOU CLOSE THE BRAKE BLEED FITTING!

5.After you close the bleed fitting, then your helper can pump the brakes up again.

6.Repeat steps 2 through 5 twice, and check the fluid level in the master cylinder.  Add fluid.

7.If you are getting any air, keep bleeding the passenger rear until no air. 

8. If no air, then you can move to the other rear wheel, and repeat steps 2 through 7.

 

 

  Keep going... as long as air is coming out, fluid should be going in to replace it.

[DanielC]

"The fluid just oozes from the rear no matter how much pumping."

 

I am thinking you might have the line plugged somewhere.   I would start by making sure the bleeder fittings are clear, by removing them and confirming you can see light through them.

 

I do not know about 620 trucks, but on 521 trucks, there is only one passage from the brake line fitting, into the slave cylinder.   In order for any air trapped in the slave cylinder to escape,  you have to have the fluid flow pretty easily back out the bleeder fitting, so when the pressure is released from the hydraulic system, the brake shoes can push the pistons in the slave cylinder back in, and then push any air in the slave cylinder back out the bleeder fitting.   If you close the bleeder fitting with residual pressure in the brake system, this air cannot escape. 

However, if you let the brake pedal up before the bleeder fitting is closed, you will suck air back into the system.

[Lee R]

I was able to bleed the MC and when I did, I got a firm pedal. This is why I think it is good and not bad out of the box. Also, when I removed the brakeline plug screws after bleeding the MC, when stepping on the brake pedal fluid would come out where the lines should be hooked up. Going to run air into the line and see what blow out.

[Lee R]

Well, it's going into the shop today to see whats wrong.  If the MC turns out to be bad, then this will be the 2nd time that Rock Auto has failed me in as many weeks.  Pretty discouraging but I thank everyones replies and suggestions. 

[Lee R]

late update....put the old MC back on and brakes work fine.  Inspecting the new MC it looks like the rear where the "plunger" pushes into the MC, the MC end is off center compared to my original MC.  Going to try to straighten it out and make sure the rubber in there is good and has a seal. 

 

Read some stuff on adjusting the "plunger" and some might need modification?

[ggzilla]

The rod (plunger) can be adjusted if it has threads.

 

Push rods without threads are adjusted by using shims.

[jeskoy]

Hey Lee, what brand MC did you buy from RockAuto?  I am about to do the same job and might steer clear of the one you picked up.  THANKS!