Brakes got mushy - troubleshooting

[Alec]

Hey guys - 2100 miles in, and I ran into my first real issue last night. I lost brake pedal resistance, and with that, lost much of my braking power. It's a 2 circuit system, with wilwood MCs and calipers. The reservoirs still contain brake fluid. Watching the pedal box as my son presses the pedal, it seems that the rear MC piston is moving much more than the front MC piston, so that would mean hydraulic pressure is lost in the rear circuit. However, I can't find any brake fluid residue anywhere. I put my hand on every fitting from the reservoir to both calipers, and they are all dry (and I checked all of the front fittings as well). How else might pressure be lost? Might the MC have failed, and, if so, how do I diagnose that? Anything else to look at? I haven't tried to simply re-bleed; I'm focused on finding on the source of the pressure loss.201060

[Mike.Bray]

Sounds like a master cylinder failure. I've heard Wilwood MCs aren't the best quality.

[rich grsc]

Check for fluid inside the footbox

[cv2065]

Older thread, but might help.

https://www.ffcars.com/threads/checking-brake-pressure.367074/

[Alec]

Check for fluid inside the footbox

Footbox is dry, both MCs are clean and dry. THe failure happened while I was driving and nothing dripped onto on my legs or feet.

[Railroad]

If it is dry under the rubber boot, the master cyl piston is by passing. This can be the seal or the check valve.
Sometimes you can stab the brake pedal and dislodge debris under the check valve.
More than likely you will need to buy a seal kit for that master cyl or replace it.
Good luck,
Let us know what you find.

[Mike.Bray]

More than likely you will need to buy a seal kit for that master cyl or replace it.

Or you can replace it with a Tilton MC and not have to worry about it.

[Jeff Kleiner]

I agree, master cylinder. With the balance bar if one master goes it will pivot over to that one and apply very little force to the other.

Jeff

[Alec]

I agree, master cylinder. With the balance bar if one master goes it will pivot over to that one and apply very little force to the other.

Jeff

Is there a way to verify that it's the MC that failed versus air got into the lines somehow? MC failure fits: When I press the brake pedal, it SLOWLY keeps dropping and dropping with steady pressure, which to me screams bypass as suggested by Railroad.

I contacted Wilwood to ask about replacement or rebuild of the MC and await their reply. Very disconcerting that the MC would fail at 2000 miles of normal road driving, if that indeed is what happened.

[Jeff Kleiner]

Very disconcerting that the MC would fail at 2000 miles of normal road driving, if that indeed is what happened.

Sometimes they fail right out of the box (like we had happen while doing a live build at the SEMA show :mad:!)

Jeff

[burchfieldb]

This is what mine looked like out of the box when I happened to pull back the boots.

201089

[CraigS]

Is there a way to verify that it's the MC that failed versus air got into the lines somehow? MC failure fits: When I press the brake pedal, it SLOWLY keeps dropping and dropping with steady pressure, which to me screams bypass as suggested by Railroad.

I contacted Wilwood to ask about replacement or rebuild of the MC and await their reply. Very disconcerting that the MC would fail at 2000 miles of normal road driving, if that indeed is what happened.
Yes it is disconcerting. But you/we will find a fix so you can move on. BTW, show us how you reservoirs are mounted so we can see height vs MCs.

[danmas]

I have a couple of wilwood master cylinders I won’t be using. <see tilton comments above>. Pm me if you want them.

[Alec]

Yes it is disconcerting. But you/we will find a fix so you can move on. BTW, show us how you reservoirs are mounted so we can see height vs MCs.

Here is an early-build pic, showing the mounted reservoirs (forte triple) at the height of the frame rail. Tops are pretty even with the rail.
201295

Here is a pic of the pedal box yesterday, from above, prior to removal of the failed MC. The rear brakes MC is on the left, closest the inboard wall of the footbox. That pushrod is straight-on with the body of the MC, so side-loading and scoring of the inner wall of the MC body should NOT have caused the bypass failure (as wilwood suggested to me might be the case, like if I had the balance bar set-up poorly and the pushrod was coming in at an angle).
201296

I removed the original, still found no fluid leaks anywhere. Replaced with the identical part that I picked up from Summit (https://www.summitracing.com/parts/WIL-260-10372), and I have solid brakes again with excellent panic braking response and no pedal fade. I will be sending the failed MC to Wilwood for them to disassemble and investigate, to decide whether they want to reimburse me the $100 for the replacement MC.

[CraigS]

Excellent. Thanks for letting us know what fixed it.