Cstoliker1

4 wheel power drum brakes on a 71 cutlass. Almost no brakes what so ever. Pedal very firm and no fluid loss. Been an on going degrading problem. Replaced master cylinder and front wheel cylinders.
With the drum off and brake pedal pressed the shoes spread as they should. Simply not nearly enough force to stop the car at any more the. 10mph.

What else could the issue be? If the vac booster was bad I would expect to loose power assist, but still have the ability stop the car with excessive force on the pedal? Is this correct or could a booster problem interfere with manual operation?

redoldsman

Did you bench bleed the master cylinder?

Cstoliker1

No. I just installed it and blead at the wheels.
However, the master cylinder was replaced a few years ago in an attempt to fix the issue (before it was as bad as it is now). It's been an ongoing problem.

Anyway, air in the system would cause a spongy pedal right?

Fun71

I once had a vacuum booster go bad and it gave a very hard pedal and little stopping power.

chequenman

You have to bench bleed your Master Cylinder. There's been lots of other people here over the years that haven't bench Bled the Master Cylinder and are in the same situation as you. Take 1/2 an hour and you will find a marked improvement.

76olds

I'm with Kenneth on a brake booster problem. Firm pedal = bad booster without a doubt.
Spongy/pedal goes to floor= air in the system or bad master cylinder.

Eric

Cstoliker1

Thanks. I'll change out the booster. I kept thinking if the booster was bad the brakes should still function if you stand on them but I guess there's more then one way for a booster to fail other then the diaphragm.
I'm reading the nuts holding the booster on are a bitch to get at. Any recommendations before I go tackle it?

tru-blue 442

You need another m/c, then bench bleed it. Then put it in the car. It should
come with instructions and plastic fittings with short tubes just for the bench
bleeding process. Good luck.

76olds

I would check the vacuum check valve between the intake and booster prior to pulling the booster, I say this because you mentioned this has been an ongoing issue with your brakes prior to the M/C change.
Also how did you bleed the brakes with such a firm pedal? Did you get any braking fade as you were bleeding with the new Master and wheel cylinders in place? Hard-spongy-fading pressure?
If you crack the line at the master does the pedal go to the floor as if bleeding at the master?
This will defiantly narrow it down to the booster as you and a couple of us assumed.
Hope this helps
Eric

Fun71

I had difficulty accessing one nut; the other 3 were fairly easy. I used an open end wrench and turned that nut one flat at a time until it was loose enough to remove with my fingers. Seems I remember I kept one of the easy to access ones snug to hold the booster in place until after I had the difficult one off.

tru-blue 442

You won't know what the problem is until you have a m/c known in working condition. If you failed to bench bleed it 1st one, you're going to continue to bang your head against a wall. Just sayi'n.

Cstoliker1

Good point.
With the bleeders open the pedal goes to the floor with little resistance. There does not seam to be any fluid restriction in the lines or mechanical resistance on the pedal.
My entire argument for the problem NOT being the booster was based on the assumption that with without power assistance, the brakes would still function to a limited extent manually. This seams to be incorrect as the breakes appear to function normally with no load. But with only the 4x mechanical advantage gained from the pedal arm/pivot by itself is not sufficient to make the brakes viable with the full weight of the car.

chequenman

Your statement above supports the theory of a Master Cylinder not having been Bench bled to give the lack of Hydraulic force and pressure to apply Brakes in the way they were designed.... You seem like you don't want to start from the beginning of your exploratory efforts to resolve a Braking problem that you were having.
Resolve your efforts by starting from the beginning by doing what most if not all Mechanics would do and follow procedures of your braking system correctly and then move forward.... There all said and so DONE.

76olds

[QUOTE=Cstoliker1;914793]Good point.
With the bleeders open the pedal goes to the floor with little resistance.

Does the pedal go to the floor even when the car is running while bleeding? If so the booster is OK.
I'm just surprised that the pedal has went very hard now being the Master Cylinder has been on the car a couple years.
I would check all the springs , shoe hold downs and shoe travel for binding.
Sometimes you need to disc grind the flats that the shoes travel on and then spread some copper anti-seize on each travel area.

Eric

Cstoliker1

The problem predates the master cylinder replacement and persisted after. Not to mention air (a compressible gas) would cause excessive travel in the pedal that could cause lack of hydraulic force when the pedal runs out of travel (hits the floor)

No one would like the problem to be solved by a bench bleed more then me. But the symptoms and history only serve to exclude the master cylinder as the problem.

Instead of blindly following procedures, applying simple logic with a basic understanding of operation can include or exclude many components or suspected malfunctions. I will properly bleed the MC to inprove performance after I get the brakes operational.

Cstoliker1

Thanks everyone. I made some real progress today. It turns out that the wheel cylinders I put in were not brand new. They were remanufactured. Upon disassembly I discovered each had warps in the cylinder walls that stopped one piston from fully extending. Effectively I was trying to stop the car with a single brake shoe on each side. That has now been corrected. The brakes are far from 100% but they can now stop the car within reason.
I'll still need to shop bleed the MC, possibly replace the rear wheel cylinders and look at the stop/check valve. The booster does appear to be oporating but it won't hold a vacuum.