Brakes - all of a sudden no stop

[matthon]

I've been driving this car with no issue for years.
Rebuilt years ago, stock sbp manual KH disc, rear drum, proportioning valve, I did upgrade to a newer style master cylinder, and a few years ago I rebuilt the calipers again for the heck of it.
Brake pedal has always been firm at the top, and gets better as it travels down.

I don't drive like a moron, always leave plenty of space between me and the person in front, however a few weeks ago I had to stop fast.

Since that moment I have a pedal that is weak with little resistance, from the top of travel to the bottom, and it will not stop.

No leaks whatsoever, calipers, wheel cylinders, lines, valve, mc, nothing.
I checked the fluid level, it is fine, I've actually haven't had the mc cap off in years and it never leaks.

Wtf happened here?

I don't want to throw parts at it, and I have some, I'd rather figure out wtf happened so I can address it, you know, as it's the brakes and all.

I was unable to locate a similar scenario, so any input is greatly appreciated.

 

[dano]

Did the pedal travel change? When you say "pedal has always been firm at the top, and gets better as it travels down" are you referring to how the bite of the brakes feel or how the pedal feels?

I'd almost say you might have glazed the front pads.

 

[matthon]

Pedal travel does not appear to have changed.
Both really, the pedal always felt like it was pushing the fluid, but I could put the pedal down farther and it would slow down more as I went.

Now the pedal is the same all the way with no stopping power at all until the very end, and it still will barely stop, not driveable.
If I pump the pedal, it feels like it might 'pump up' like there is some resistance, but there is no change.

 

[dano]

Pedal travel does not appear to have changed.
Both really, the pedal always felt like it was pushing the fluid, but I could put the pedal down farther and it would slow down more as I went.

Now the pedal is the same all the way with no stopping power at all until the very end, and it still will barely stop, not driveable.
If I pump the pedal, it feels like it might 'pump up' like there is some resistance, but there is no change.

With that I think getting the front on stands and watching the brake actuation (with a helper in the car) and observe each piston. I had a floating caliper seize and it felt how you describe.

 

[Righty Tighty]

Another thing to consider is the possibility that the cup that separates the fluid between the front and rear reservoirs is bad. You'll get little to no braking and no signs of leakage, because the fluid is being pushed from one reservoir to the other.

 

[340demon72]

If no external leak the master cylinder piston is not pushing fluid due to internal leak/bypass (bad cups or bore). Also if pedal doesn't return all the way to the top the pistons cannot replenish fluid through the replenish port or it is blocked with sludge/ debris.

 

[furyus2]

You never said whether you actually checked the master cylinder fluid. If you didn't, start there.

 

[Murray]

M/C.

 

[matthon]

I have a new mc in stock, so I'll try that, bleed everything.
Thank you fabo!

 

[matthon]

New mc installed, bled the brakes completely until ran with new fluid.
Rears bled fine, front right was a bunch of foamy fluid for a while, but after that the pedal came back to normal and the front left was normal.

Also, both front right caliper bolts were loose, and the front left caliper was tight but was missing a bolt.
I recall a few months ago it sounded like we hit something and threw it up against the floor, but didn't see anything on the road before or after. Stopped and checked out the car, and when we got home thoroughly inspected it but definitely did not see a caliper bolt missing - maybe it was gone and just didn't see it not there.

Anyhow, brakes are better than ever.