Brakes needing to be pumped after new master cylinder

[xflyer]

Turns out it was a problem with the calipers, but not the way I had theorized. This type of situation, calipers on opposite sides, is why in aircraft maintenance a second or even third set of eyes is on the procedure before things go back into service.
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1989 Campmobile, 1984 7 passenger beater Vanagon
Both with cool A/C

[MarkWard]

Thanks for the update. The experience curve can bite at times. Probably should thank Go Westy for going above and beyond with you. That has been my experience.

[brewvan]

Bone head move of the year. I installed the calipers on upside down… bleeder screw was lower than the trapped air in the cylinder. Brakes are working great now. Thank you all for the help!

[Nekobus]

Just wanted to say that I was having a similar issue in my 90 Multivan and a new brake master cylinder solved the problem.

[xflyer]

A possible clue here is the report that installing the pistons into the calipers was difficult. The square profile O ring caliper/piston seal may have been distorted. In normal operation the seal deflects when the brakes are applied and when the pedal is released the seal returns the piston into the caliper. That's so the pads do not drag excessively on the rotor. If the seal is distorted the piston may not extend correctly.
Try this: With wheels off the ground have someone push on the brake pedal while you spin the front wheels. See how long it takes (pedal travel or pumping) to stop the wheel. Then let off the pedal and watch the pads and see how far they retract. Should be barely visible, but the wheel should turn freely, there will likely be some rubbing of the pads on the rotor, as long as there is no excess drag its OK.
If the pistons retract a lot the calipers may need repairing. May need new O rings.
_________________
1989 Campmobile, 1984 7 passenger beater Vanagon
Both with cool A/C

[Wildthings]

A soft pedal can be caused by air in the system or the drum brakes not being adjusted up tight enough. Pressure bleeding can help in getting any air out. If there is a defect inside the master that could cause the pedal to sink. Sometimes a master will have a piston seal that will leak past under low pressure but once you get enough pressure the leak will stop so long as the pressure is applied.

[raygreenwood]

[MarkWard]

Do you have some large C clamps? You could pull the shoes temporarily and clamp both wheel cylinder pistons compressed completely. See how the brake pedal feels. This would avoid having to open the hydraulics and might reveal something.

[brewvan]

I can pump the brakes up with it not running. Takes about 3-4 pumps and they are solid and responsive. If I hold the brake pedal down and fire up the engine the pedal sinks to the floor pretty quickly. When it’s running I have to pump them fast to get it to stop but it doesn’t feel like it holds the pressure after pumping it up for long.

[MarkWard]

The pedal is fine with the engine not running and you’ve removed the stored vacuum? Only sinks when engine is running? Or did I miss that?

[?Waldo?]

If the calipers are not leaking, they are not the reason the pedal goes to the floor.

[brewvan]

My brake booster looks like it’s what ever came stock in the 86 gl westy. It works for sure. When the engine is running the pedal sinks to the floor. I’m gonna rebuild the calipers again with the brake paste.

[MarkWard]

A page back I asked what engine and what brake booster you have. I don’t see that answered.

When you reseal calipers you need to apply a brake paste to ease assembly on the square seal and the piston. The piston should move with resistance but freely.

[brewvan]

Wheel cylinders are new. No leaks have been found anywhere. I rebuilt the calipers and when I was putting them back together it was very difficult to get the cylinder back in. But they did go in. Not sure if that would have anything to do with this. Every time I bleed it no air comes out even after pushing a quart through each bleeder screw. Everything is new in the system except the pressure regulator, calipers(rebuilt), and shoes/drum/hardware(excluding cylinders). I’ll try the bb trick and isolate the systems when I install the replacement MC. But I’m at a loss here for what to do.

[MarkWard]

A BB is the ammo for a BB gun. It’s a tiny ball bearing that you shoot out of a gun.

You need to isolate front or rear. Block one circuit at the master and test. Yes it’s a messy job disconnecting and reconnecting the brake lines, but you should be quite good at it by now.

Edit: short of having BBs or making your own master cylinder plugs, these should work for testing.
https://www2.cip1.com/c13-18-1103/?srsltid=AfmBOopXioUi3nCc7zUqfKQgsnwENMFsapBlVKJZ62YSxGLcns3XjCtq

[xflyer]

"Now that it’s not the MC what should I start to suspect?"
Check the rear wheel cylinders. May need to remove drums and peel back the boots. Look all along the lines and hoses for leaks.
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1989 Campmobile, 1984 7 passenger beater Vanagon
Both with cool A/C

[Wildthings]

[brewvan]

What do you mean by BB’s? Also I sent my old MC into GW. They tested it and didn’t find any issues. Out of courtesy they sent me a new one. Now that it’s not the MC what should I start to suspect?

[?Waldo?]

^^^^ Very good advice! ^^^^

[MarkWard]

If you have a couple BBs, remove the two lines from the master and temporarily install the BBs. Bleed what little air is in the master by pressing the pedal with the fittings cracked open. Then snug and repeat. This should isolate the master and booster from the hydraulic system. If you still have a sinking pedal, the master is suspect. If the pedal is solid, you have another problem.

You can also make master cylinder plugs with a short section of brake tube. Cut it in half and fold the cut ends to crimp off the flow. Install in the master and bleed as above.