1968 brake conundrum

[Jared Roy]

Hey guys and girls. I have 5/68 baywindow bus with fresh wheel cylinders, lines, and master cylinder done recently. I went to stop the other day (on flat ground, thank goodness) and brake fluid started spraying out of the reservoir cap all over the underneath of the dash. No brakes whatsoever. Has this happened to anyone before?

[busdaddy]

That's a new one, apparently you have issues in the master cylinder. Was it rebuilt?, or a new replacement?
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[aeromech]

How about a couple pics of your installed master cylinder?
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[Jared Roy]

It's a German made new replacement. I don't have any pictures. I've had many buses and nothing like this has ever happened. I've owned this one for 20 years. I'm gonna pull it off when I get a chance. Just wanted to know if anyone has had a similar experience. Thanks.

[aeromech]

Never. I just thought maybe someone plumbed it wrong
_________________
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

[SGKent]

that is a new one.sounds almost like it injected compressed air in the system.
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[raygreenwood]

That effect is commonly caused by either a compensation port flap valve either bent, cracked or propped open by crud.... letting high pressure back into the reservoir on the pressure stroke.....or a flap valve combined with a cup seal that is also bypassing on the inisde where it contacts the piston (sloppy fitting seal)....and usually coupled with a fluid reservoir that is overly full.

Usually if the fluid fill level is correct....which is not up to the brim.....a back pressure surge crom an internal defective flap valve or piston seal....does not move enough volume to force flujd out the top. The piston circuit volume is just not that big.

And....with the piston stroke pressure venting through a conpensation port.....there is no pressure building on the circuit so you lose brake pressure.

The risk of propping a flap valve open or cutting a cup.....with a simple grain of grit....is why every single brake hydraulic part on brand new assemblies gets stripped and cleaned with denatured alochol. You would be amazed at the crap that comes out....even on German parts. Brake cylinders (short of anti lock brake metering blocks) are not assembled in clean rooms by people with white gloves. Ray

[Abscate]

Brake conun - drum indeed.
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[Jared Roy]

Thanks for all of the replies. I can't imagine why after only 500 or so miles it would decide to get air or crud in it. I was very generous with the brakleen and air compressor with the super jet on it. I'll get to the bottom of things when I get it apart. Once again, thank you.

[raygreenwood]

[Jared Roy]

No no no. Not the corrosive crap, and not internally. Just made sure there was no road crud that could get inside.

[Wildthings]

If one of your brakes is dragging and you boil the fluid in one of the slaves this might happen.

[Jared Roy]

Thanks Wildthings. I considered that but none of the brakes are dragging.

[raygreenwood]

Good to hear you kept the brakleen out of the guts.

I would carefully....on a layer of paper towels....disassemble the MC.....looking at what floes out of it. Look carefully at each piston begore pulling anythi ng furthet apart and see if it appears that any of the little brass flap valves that are between the cups and pistons....appear to be lifted away from the piston head surface

Also see if there is excessive slack on the piston of any of the cup seals.

Thoigh you may or may not be able to fix it.....its nice to actually find whats wrong. Ray

[Jared Roy]

I'll do that. Thanks for the advice Ray.

[SGKent]

I think you have air in the brake lines. Try adjusting the brakes and bleeding them with a pressure bleeder.

The compensating port gets covered with the piston cup seal. There is no "flap." Brakekleen can be used on the metal parts to clean them however it has to be the zero residue kind and compressed air must be used to get all of it out. As Ray said it CANNOT get on the seals, nor can there be any residue left to get on the seals. Generic parts cleaner cannot be used either because it leaves a residue. Then let it sit in the sun for awhile or on the work bench to Make sure all has evaporated. I've seen some folks use really hot soapy water but the drying process has to be instantaneous or flash rust forms. I'd say use brake fluid to wash it out but I once ruined a good set of calipers when it stayed s couple days in high humidity weather and the remaiining fluid, which is hygroscopic, pulled enough moisture out of the air to cause rust pits inside the calipers.

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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[raygreenwood]