1971 bus not stopping after full brake job

[airschooled]

[skills@eurocarsplus]

Ok....seeing you did replace everything here is a theory....

How is the brake pushrod adjustement IN the booster? Aftermarket boosters are junk...and even OE. Ones may have been fooled with. If the adjustment is too long it will compress the mc piston somewhat and you won't get a "full shot" or stroke in the bore....that may be your whole problem.

Sounds like you have run enough fluid thru it. I never bench bleed masters....pressure bleed here or pump n hold....

Neve had any issues vaccum bleeding either. In fact it works well. I suspect the pushrod or a bum master.

If your pedal is rock hard with the engine off chances are there is no air, and its the master or pushrod adjustment.

Did the mc go on the booster easily? Or did you have to "draw it in"? If you did, that is a sign the pushrod is too long
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[lil-jinx]

Vacuum bleeding becomes problematic to me when we use negative pressure to bleed a system that spends its life at 10psi and above. What part of the vacuum bleeding process gets the residual pressure back to operating pressure? (The wheel cylinder/shoe springs of drum brakes and residual valves in the MC keep about 10psi in the lines at all times. Vacuum bleeders can't contribute to this.)


If your brake system is retaining pressure,then you have a problem,the pressure applied to the wheel cylinders by the brake shoe return springs forces the fluid back into the reservoir.If there was 10 psi in the system and you were to remove one set of shoes the pressure would push the pistons out of the wheel cylinder,it would also cause the brakes to drag.
The front disc brake caliber piston has about 6 square inches of area,10 psi would apply 60 pounds pressure on the brake pad,and that would cause the m to drag.
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1974 Bay Westfalia. she likes to be called Ethel.
1968 Dune Buggy
1974 parts bus
A smart man learns from his mistakes,a very smart man learns from other's mistakes.

[Abscate]

[SGKent]

[timvw7476]

said bus could've been gravity bled 23 times since I posted...
yaaaaaaayyyy........ gravity.

[Tcash]

Brake booster pushrod adjustment
Brake Booster


What part of the vacuum bleeding process gets the residual pressure back to operating pressure?
It doesn't, the first application of the brake pedal restores the residual pressure.

Once a master cylinder is bench-bled, there is no functional difference between pedal bleeding and pressure bleeding.
There has to be. Because some vehicles you can't get the air out without pressure bleeding.

If your brake system is retaining pressure,then you have a problem,the pressure applied to the wheel cylinders by the brake shoe return springs forces the fluid back into the reservoir.If there was 10 psi in the system and you were to remove one set of shoes the pressure would push the pistons out of the wheel cylinder,it would also cause the brakes to drag.
The front disc brake caliber piston has about 6 square inches of area,10 psi would apply 60 pounds pressure on the brake pad,and that would cause the m to drag.
Residual valves are used on drum brakes not disc brakes.
http://ratropa.com/mc.jpg

said bus could've been gravity bled 23 times since I posted...
Gravity bleeding is a good way to exchange the fluid, before pressure/vaccum bleeding. But it does not move all the air bubbles.
This can be demonstrated by observing an air bubble in a clear liquid tube. The fluid will drain past the air bubble but the air bubble will maintain its position or move up the tube.

[airschooled]

My mistake- the drum brake buses have residual (10psi) valves on all wheels and the 71+ buses don't even have residual valves on the rear wheels. I wonder what they were thinking, considering most cars have 10psi residual valves for drums and 2psi residual valves for discs.

Robbie
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[aeromech]

WTF is a residual valve? If you're talking about wheel cylinders, I've rebuilt several and they consist of a bore, piston, and cup seal. Return springs on the shoes might keep some pressure in the brake lines but I surely wouldn't call a wheel cylinder a residual valve. There doesn't seem to be any difference to speak of between a 1971+ wheel cylinder and an early cylinder. At least not that I've ever noticed.

To the OP,
Did you buy a NEW booster? Did you follow the directions and adjust the rod end on the aftermarket booster per the instructions? Can I have your original booster?
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Lead Mechanic: San Diego Air and Space Museum
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Boeing 727,737-200-300-400,757,767
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VW Type 2 1971 (3 ea.) 1978, 1969
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[Tcash]

[lil-jinx]

It's a good day,I learnt something,residual pressure valve,Thanks Robbie.
_________________
https://www.thesamba.com/vw/forum/viewtopic.php?t=620186
https://www.thesamba.com/vw/forum/viewtopic.php?t=668799
1974 Bay Westfalia. she likes to be called Ethel.
1968 Dune Buggy
1974 parts bus
A smart man learns from his mistakes,a very smart man learns from other's mistakes.

[aeromech]

Me too. Thanks Robbie
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[skills@eurocarsplus]

his 71 doesn't have these.......
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[airschooled]

[TomWesty]

I just replaced my MC and Servo in my '71 in July maybe. Used the new Airtec servo which has worked quite well. I just adjusted the push rod to protrude from the body of the servo the same length as the push rod protruded from the old servo. I used the Brasilian MC on Aeromechs recommendation. I tried to vacuum bleed it and that literally and figuratively sucked. Went to the old school pump the pedal method and it worked fine. The idea is to get a bubble free stream of fluid that extends all the way from the reservoir through the MC out to to the bleeders and then close the bleeders. Gravity, pressure, pump, doesn't really matter unless you are in a time is money situation. Then probably a pressure bleeder would be good.
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[Abscate]

I think hell is a continuous loop of brake bleeding threads....where Is TK when you need him?
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