78 Bus Poor Braking

[blatzer]

there is a new master cylinder in my 78 campmobile, i bought it 1,2 years ago and installed it 3 weeks ago- all was well, i thought, until over the past few days the van stopped stopping in a timely fashion-- push rod tube adjustment, done; brakes bled, done, but an indicator of the problem at hand, because during the bleeding process, the fluid can be seen to drain from the reservoir, but upon release of pedal some of the fluid returns- the other symptom i notice is initially upon braking, there is a slight grab of the brakes, as if they're going to work, but as more pedal is applied the stopping power is diminished, and ultimately the van stops far too slowly to be safe, it feels like the fronts (disks) are not working because if i really hammer the brakes the back will lock or skid-- the calipers, pads, rotors, drums shoes, etc are in good shape.....

[Amskeptic]

there is a new master cylinder in my 78 campmobile, i bought it 1,2 years ago and installed it 3 weeks ago- all was well, i thought, until over the past few days the van stopped stopping in a timely fashion-- push rod tube adjustment, done; brakes bled, done, but an indicator of the problem at hand, because during the bleeding process, the fluid can be seen to drain from the reservoir, but upon release of pedal some of the fluid returns- the other symptom i notice is initially upon braking, there is a slight grab of the brakes, as if they're going to work, but as more pedal is applied the stopping power is diminished, and ultimately the van stops far too slowly to be safe, it feels like the fronts (disks) are not working because if i really hammer the brakes the back will lock or skid-- the calipers, pads, rotors, drums shoes, etc are in good shape.....

HELLO blatzer, I suspect your booster. Go to a parking lot without cars in it and shut off the engine at 20 mph, but please don't turn the key any further and lock the steering wheel. See how the brakes apply without the booster. I expect they will feel just like a pre-1971 bus, heavy but smooth. Then compare with the engine running this time when you stop from 20 mph. The booster gives the initial assist, but if it is not getting new vacuum quickly enough, you will have a stiffening pedal within a second or two. Check your brake booster-to-master cylinder seal. I bet it is leaking. It has to be perfectly air-tight. I have used two seals there in the past to overcome rust.
Colin

It is normal for the reservoir to take in fluid as the shoes/pads retract.

[blatzer]

thanks for the word, 'skep, i'll check it out- the reservoir issue caught my attention, because of the reduced volume per pump vs my previous experience/memory about how much of the fluid was dispensed, and then my murphy reflex thinking i have the wrong part installed

[blatzer]

just bought another master cylinder, after bleeding, the same problem, no pedal pressure on the front brakes

[Ryno]

Do your calipers have one or two bleeders?

[Amskeptic]

just bought another master cylinder, after bleeding, the same problem, no pedal pressure on the front brakes

What is "no pedal pressure on the front brakes"? The pedal will give you resistance somewhere, either bottomed-out or up-firm or anywhere in between. Do you have no braking at the front and you lock up the rears?
Bleed the entire system carefully and correctly. We are not going to blame the master cylinder and would not have blamed it the first time through.

Do you still have what feels like a momentary boost when you first apply the brakes that gives out and makes a firm non-boost pedal? Which wheels lock up when you hit the brakes firmly and smoothly.
Colin

[blatzer]

2 year (at least) shelf life ate master cylinder= bad, new chiny master cylinder (yikes!)=leaky, trw on order, maybe this will be the final post on this thread