Brakes all replaced but master cylinder still having issues.

[Krex]

Brake problem this is a 1970 k20 lwb with drum all around.

Replaced all wheel cylinders, rubber hoses to cylinders, replaced all shoes,

Reused hardware where I could no leaks that I see in the system bleed brakes in correct order , feel pressure on brake pedal, but still spongy but before had nothing, truck truck on and put it into reverse and I can not stop truck put it in drive can not stop truck,

I think the last thing to replace is master cylinder but, it holds fluid and I see no leaks

What should I do next????
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[Wrenchbender Ret]

It must have power brakes. Is it getting vacuum to the power brake booster? You should replace master cyl. when doing extensive brake work like that. It is working in territory that was not used & is full of crud.

[MagmaJct]

Having recently rebuilt the master cylinder on my '71, I have a better understanding of how the hydraulic system works.

You could have a worn/failing master cylinder that has an internal leak. That's right, an internal leak. Think of it as the brake's version of "internal bleeding" like what can happen with us humans.

The piston(s) that move the hydraulic (brake) fluid down the lines also has fluid behind it. This fluid behind the piston is not generally pressurized, but it has a direct path to the reservoir.

If the piston seal is worn, fluid will move from the pressurized side of the piston into the unpressurized and return to the reservoir. No air is introduced into the system.

This is what happened to my '71, which does not have vacuum assisted brakes. I'd stop at a light. While waiting, the brake pedal would slowly continue to lower until the truck would start to move forward! As long as I kept solid pressure with my foot, everything worked fine.

When I rebuilt the cylinder, I compared the old seals to the new. The "cup" that fits tight against the cylinder wall was considerably thinner on the old seals. Over the years, the rubber will wear down.

Could this be your problem? I'm not sure. Good Luck!

Regards,
Mike

[Krex]

Should have pulled my master cylinder first I think this it my problem or looked under dash. After pulling I can tell the seal looks worn and leaking a little fluid.

Thanks guys.

I'll let you all know after I install a new master cylinder. Hopefully I will be able to have brakes soon.

[Krex]

Ok new master cylinder installed no booster. And bench bleed first.

Same issue.

Let me be more descriptive on symptoms.

Pull my foot on brake pedal brake falls with a little force springs back up but prob mostly to the spring itself.

Turn truck put in reverse moves on own, stopping only by in gear park same In drive. Any help or clues?

[ho70]

Power booster leak in diaphragm, so when you push on pedal the diaphragm fails

[Krex]

No peer brakes just master cylinder.

A still need some help.

[88Ironduke]

Did the problem with no stopping power begin before or after the brake shoe replacement at all four corners?

Ensure the brake lines are bleed out, when in doubt do it again. Then look at the brake adjustment. If the shoes are not adjusted properly you will beat your head on the wall and empty your wallet changing parts that were fine. Drum wheel cylinders have small amounts of movement compared to disc set ups. Even a minor amount of slop will result in inoperative/weak drum brakes. If the master cylinder is old go ahead and change it. Peace of mind can be purchased... Bench bleed, 4 corner bleed, re-adjust all drums. Be objective about your own work and nail down each part of the puzzle.

Good luck
Paul

[motorcritter]

Adjusting drum brakes can be a dark art, and getting all the air out, after a major rework can be tedious. I've used a hand pump to push fluid up through the system on tricky jobs and motorcycles. I've also rigged up a pressure setup, using my radiator leak tester, to bleed out ABS systems (the pump sections can get full of air bubbles, and without a vehicle-specific scan tool, manipulating the valves can be impossible)- pressurise the system and open it up, each line at a time. Good luck, and hope that helps.
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