Rep Power: 0

This is a friend's car, and I'll describe the recent history as best as I can.

99 civic lx 4dr, non-abs. There was a small leak in one of the rear brake lines, but it didn't noticeably affect braking performance. Friend had the line repaired, and then for some reason was told the MC was bad and needed replacing as well (maybe they bottomed out the pedal while bleeding?). Had the MC replaced, and wasn't able to restore proper braking.

I'm now trying to have a go at it. The pedal goes to the floor easily, and cannot be pumped up. It does firm up without vacuum assist, but I'm not sure it's as hard as it should be. It does brake mediocre enough for cautious driving, but one emergency and it'd be game over. Doesn't feel like it's biasing one circuit over another, but again, hard to say.

• Confirmed there's no longer any leaks.
• Parking brake is nice and firm, locking both drums up after a few clicks. I haven't pulled the drums, but since it was fine before the car was worked on, I imagine the star wheels are adjusted fine and don't suspect an issue here.
• I have a Motive power bleeder, so I got the large universal adapter, and ran a quart through all 4 corners at 15psi, working farthest inwards. Unfortunately, didn't find any air.
• I only have one clamp suitable for the brake hoses, so I tried clamping off one at a time to see if I could isolate the issue to a corner. No dice here either.
• I need to examine the front brakes a little more, but again, if it was fine before the recent repairs, I can't imagine a new issue forming. And it seems unlikely there'd be a simultaneous issue with both sides, so if only one corner had an issue, clamping it off should've firmed the pedal right up.

Seems to be pointing back at the MC being at fault. Maybe the person who installed it didn't properly bench bleed it? Would pressure bleeding at 15psi take care of the MC? After I did all 4 corners and saw no air or change in pedal response, I tried again, pumping the pedal while pressure bleeding, hoping it'd have an effect on bleeding the MC in place. Is this a viable alternative to undoing the lines at the MC and bleeding it directly?

Any other tests I can do? Or something to definitively rule the new MC bad, instead of trying to throw parts at it (ie another MC)?

Thanks

 
Rep Power: 223

Try bench bleeding the master, then bleeding the car again. Fluid is cheap. I personally alway do the two man method instead of the power bleeder.
it is possible that the new master is bad
100% sure there's no leaks? That stupid plastic cover on the underside can hide alot.

 
Rep Power: 0

I'll have to take a better look down there, but I'm fairly certain. Had to drive it 15 miles to get it to my street, and the reservoir level didn't dip. No spots on the ground, but I'll obviously have to find and check that cover.

The pedal behavior still feels strange to me. Normally with air in the system, I'm used to the pedal sinking straight to the floor without any resistance, and possibly being able to be pumped up. No pumping works here. But the pedal actually reliably firms up for the last bit of travel (and actual braking occurs). Any chance the wrong version MC could cause this (ABS vs non-ABS variant-- I know the ABS has a bigger bore)?

Because of time constraints, I got the go-ahead to get the cardone reman for $28 on rockauto. So much for not throwing parts at it, but short of there still being a leak in lines, it seems I'd be attacking the MC one way or another. Figure this way, I get to skip another $10 quart of brake fluid if bench bleeding the current MC wouldn't do the trick.

edit: No markings on the MC, but I did check under the car. Looks like whatever plastic covering was cut and removed. I can see the hard lines end to end, including the new section-- all dry.

 
Rep Power: 223

I personally would not get Cardone, they tend to be junk. Try to get like centric, or raybestos

 
Rep Power: 0

Yeah, I would've gotten something better if it were my car. Got the go-ahead to cancel and upgrade to a raybestos. Do they usually come with the fittings/hose to bench bleed? I haven't done an MC in a decade, and last time, I just did it in a tub

 
Rep Power: 223

Never seen a master cylinder come with bench fittings. Just make some up or let the fluid spill into a catch tray

 
Rep Power: 0

I'm out of ideas.

So the raybestos mc came in today. Came with nylon bolts for bench bleeding. Did that, and pulled out the "old" one to compare. Same part. Same exact behavior. So the recently replaced mc seems to be in working condition and as bled as the new one. Proceeded with the swap. Hooked up the motive power bleeder, and bled at 15psi. Exact same behavior as before. Pedal firms up properly without vacuum assist, but easily strokes to the bottom with vacuum. Cannot pump up.

Pulled both drums to examine shoes and wheel cylinders. No leaks. Car has 130k mi, and everything in there looks very new. Confirmed again no leaks.

Rotors are rusty, and while I don't have a caliper to measure width, I'd imagine they're well above threshold:



Pads are at about replacement thickness, but I can't imagine they're the culprit:



Again, braking was completely fine before my friend had the car worked on (line repair + new mc). Any ideas? Thanks

 
Rep Power: 223

Bad booster maybe?
if you leave the vacuum line off the booster it works normally?

 
Rep Power: 0

Hmm that's not something I had considered. I thought a bad booster would just result in no vacuum assist, or a vacuum leak for the engine? Will test tomorrow

 
Rep Power: 223

They can fail in all sorts of ways, discovered the one in my truck is bad, below -25* I had no assistance and the pedal went to the floor

 
Rep Power: 0

Looks like I found the culprit. Pulled a caliper and found the piston rubber boot looking like a balloon. Somehow it maintained enough of a seal to keep all the leaking fluid inside.

 
Rep Power: 368

glad you found the culprit!
working on cars are really not straight forward, the common answers do not always apply and need to disassemble, inspect...