Bleeding Brakes - Need help confirming bad master cylinder

[IowaBen]

Need some help diagnosing a brake bleed that isn't working. First timer here, so that may be the issue.

I have separate reservoirs for front and rear. Lids off both.

I tried a bench bleed, which I understand to be disconnecting the brake lines and running a short line to dump back into a reservoir. The rear MC wouldn't push anything through, and the front one seemed to be working. I hooked the brake lines back up, and tried to bleed the rear and nothing came through - not surprisingly. I also tried a vacuum pump from harbor freight, and still nothing.

I moved on to the front (since I had a loose fitting and a leak). I was able to bleed the front brakes no problem.

Pressing the pedal, both MC's are being articulated also.

Seems like maybe the rear MC is bad. Thoughts?

[Rdone585]

Since you appear to have a dual MC from your description, if you also have a balance bar to set brake bias front-to-rear... is it possible the bar is set to far to the front? if you are able to manually push the MC piston for the rear system, can you see fluid movement? Perhaps a picture or a more detailed description of the MC would help focus the discussions.

[IowaBen]

I have the wilwood pedal setup that came with the complete kit.

The piston moves when the pedal is pushed, and also when I manually push it in. Neither scenario pushed fluid through.

153403

[NAZ]

Some things to check:


· * Make sure the balance bar gaps are set correctly as detailed in the instructions.



· * Ensure your M/C piston is returning completely home when you let off the pedal. There is a port in the M/C that must be uncovered for fluid to enter the M/C and that requires the piston to be fully retracted. An incorrect installation will not allow the piston to fully retract. There should be a slight bit of free-play in the push rod when the pedal is fully home against the stop. If the pedal does not return completely against the up stop the piston may not be returning fully home and that port will stay covered.

·
* The piston should have full travel available. Some installations have a frame member blocking the pedal preventing full travel.

·
* Are the bottoms of your reservoirs above the inlet of the M/C’s?

·
* The reservoirs must be vented to atmosphere. The vent is typically a very small hole in the cap that can get plugged by debris. The better ones have an elastomeric bellows like rubber seal that vents the space between it and the cap to atmosphere but still use a small vent hole in the cap.

[IowaBen]

Thanks - I've double checked all these things, and none seem to be the culprit. I've also confirmed the fluid is making it to the MC.


Some things to check:


· * Make sure the balance bar gaps are set correctly as detailed in the instructions.



· * Ensure your M/C piston is returning completely home when you let off the pedal. There is a port in the M/C that must be uncovered for fluid to enter the M/C and that requires the piston to be fully retracted. An incorrect installation will not allow the piston to fully retract. There should be a slight bit of free-play in the push rod when the pedal is fully home against the stop. If the pedal does not return completely against the up stop the piston may not be returning fully home and that port will stay covered.

·
* The piston should have full travel available. Some installations have a frame member blocking the pedal preventing full travel.

·
* Are the bottoms of your reservoirs above the inlet of the M/C’s?

·
* The reservoirs must be vented to atmosphere. The vent is typically a very small hole in the cap that can get plugged by debris. The better ones have an elastomeric bellows like rubber seal that vents the space between it and the cap to atmosphere but still use a small vent hole in the cap.

[edwardb]

If it won't pump fluid and you're positive the pedal is not being stopped before the pushrod in the MC hits the snap ring stop, then likely the MC is defective. Rebuild on those is really easy with a < $30 kit. Replaces every part except the housing itself. Including the seals which are the likely culprit. Been there done that. Or just replace it completely.

[J R Jones]

NAZ & eb, your recommendations are spot-on. Not retracting enough to expose the fill port is an easy error.
In the case of the bench bleed I am concerned. I speculate that without a seal the piston would still move some fluid.
Could there be a missing or incorrect machining step?
Could an unconventional test prove/disprove?
How about disconnecting the out fitting and blow air through cap/hose to see if the gas (air) passes through the M/C bore?
jim

[Rdone585]

Jim, I agree that's actually a good test that hadn't occurred to me. You might also be able to use a stiff wire to probe the exit hole for obstructions.

[IowaBen]

Thanks for the ideas everybody. I did order a rebuild kit today, but I'll pull the MC off the car this weekend and try blowing air through it.