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Brake failure! Is it the MC?

D

DWinTX

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Had a scary thing happen yesterday. My son was driving his 65 Coronet when the brakes failed. All of them, not just one end. Fortunately, he was almost stopped already at a stop sign when it happened. Pedal went to the floor and the brakes were 95% gone. They provided a little drag so he was able to limp it into a parking lot and called for a tow.

There was no fluid loss. We looked at the street and it was clean. When we got the wheels off we checked the entire system, no leakage anywhere. MC was full. But the pedal just went to the floor with minimal resistance.

This has had the Disc-O-Tech swap so manual front disks with 70's A body stuff on the front. Rear drums are stock style, but rebuilt. The MC is a Raybestos MC36406 (15/16" bore). I happened to have a spare MC (same part #), so since we couldn't find any leaks, we swapped it on. With just bench bleeding there was plenty of pedal pressure.

This brake system, including the M/C, probably has 500-600 miles on it. I pulled the piston out and the seals look fine, I see no tearing. This is supposed to be new, not reman. The bore looks good as far as I can tell. I'd think if it was scored, out of round, whatever, that the brakes wouldn't have worked from the start.

What could be wrong with it? Have you ever had an MC fail on both sides? I thought they we designed not to do that?

These brakes worked great for those 600 miles, didn't even notice they weren't power. The pedal didn't require much pressure and the stopping power was as good as a modern car.
 
I'll be watching this thread as I have the Dr Diff set up with 15/16 master cylinder. I agree that pedal pressure is light for manual brakes, but full pedal travel is long.

Out of curiosity, did you refill the suspect master cylinder and try bench bleeding again to replicate the symptom?
 
There was no room to add anything, it was still full. I would have thought the seals had torn, but they look alright. I don't know how they could suddenly just not fit the bore tightly.
 
Oops, sorry, I misread your question. No, I didn't bleed and try the original MC again. Guess I should try it.
 
DWinTX said:
Had a scary thing happen yesterday. My son was driving his 65 Coronet when the brakes failed. All of them, not just one end. Fortunately, he was almost stopped already at a stop sign when it happened. Pedal went to the floor and the brakes were 95% gone. They provided a little drag so he was able to limp it into a parking lot and called for a tow.

There was no fluid loss. We looked at the street and it was clean. When we got the wheels off we checked the entire system, no leakage anywhere. MC was full. But the pedal just went to the floor with minimal resistance.

This has had the Disc-O-Tech swap so manual front disks with 70's A body stuff on the front. Rear drums are stock style, but rebuilt. The MC is a Raybestos MC36406 (15/16" bore). I happened to have a spare MC (same part #), so since we couldn't find any leaks, we swapped it on. With just bench bleeding there was plenty of pedal pressure.

This brake system, including the M/C, probably has 500-600 miles on it. I pulled the piston out and the seals look fine, I see no tearing. This is supposed to be new, not reman. The bore looks good as far as I can tell. I'd think if it was scored, out of round, whatever, that the brakes wouldn't have worked from the start.

What could be wrong with it? Have you ever had an MC fail on both sides? I thought they we designed not to do that?

These brakes worked great for those 600 miles, didn't even notice they weren't power. The pedal didn't require much pressure and the stopping power was as good as a modern car.
Click to expand...
Your master cylinder is a brake cylinder in reverse - it exerts the pressure rather that receiving it. For it to fail this way only two things could have occured: 1- it was not exerting forward pressure which means the seals flipped or the seals somehow were not hugging the wall of the bore or the stem of the master shaft, and, 2- whatever fluid it took in from the reservoir to exert pressure, it put right back in the bowl when the pedal was pushed. In other words, instead of the fluid going down the lines it headed right back in the bowl. All that's left is the mechanical side of things but you've already proven that's not the issue.
 
Yatzee said:
2- whatever fluid it took in from the reservoir to exert pressure, it put right back in the bowl when the pedal was pushed. In other words, instead of the fluid going down the lines it headed right back in the bowl.
Click to expand...

What would cause this? Anything I can do with the MC off the car to test if it's doing it?
 
Yatzee said:
the seals flipped or the seals somehow were not hugging the wall of the bore or the stem of the master shaft
Click to expand...
Would brake fluid leak out the plunger inside the car if this happened?
 
I didn't notice any fluid in the pass compartment, on the pushrod, or the back of the MC.
 
are you able to pump up the pedal until its somewhat firm? if so
you got traped air in the system. have you did the two person bleed session starting at the rear passenger wheel.
 
DWinTX said:
Had a scary thing happen yesterday. My son was driving his 65 Coronet when the brakes failed. All of them, not just one end. Fortunately, he was almost stopped already at a stop sign when it happened. Pedal went to the floor and the brakes were 95% gone. They provided a little drag so he was able to limp it into a parking lot and called for a tow.

There was no fluid loss. We looked at the street and it was clean. When we got the wheels off we checked the entire system, no leakage anywhere. MC was full. But the pedal just went to the floor with minimal resistance.

This has had the Disc-O-Tech swap so manual front disks with 70's A body stuff on the front. Rear drums are stock style, but rebuilt. The MC is a Raybestos MC36406 (15/16" bore). I happened to have a spare MC (same part #), so since we couldn't find any leaks, we swapped it on. With just bench bleeding there was plenty of pedal pressure.

This brake system, including the M/C, probably has 500-600 miles on it. I pulled the piston out and the seals look fine, I see no tearing. This is supposed to be new, not reman. The bore looks good as far as I can tell. I'd think if it was scored, out of round, whatever, that the brakes wouldn't have worked from the start.

What could be wrong with it? Have you ever had an MC fail on both sides? I thought they we designed not to do that?

These brakes worked great for those 600 miles, didn't even notice they weren't power. The pedal didn't require much pressure and the stopping power was as good as a modern car.
Click to expand...
The rearmost cup is not the cup that exerts pressure, rather it sucks the fluid into the piston and acts as an end seal. If any of the cups forward of this one failed ( rolled, folded back at the outer lip) you won't have fluid out the back because there's no pressure present here to cause a leak. When you pulled the piston out, the cups may have folded back to their normal position as they dragged against the piston wall. If there are check balls built into the casting, the fluid probably just moved back and forth from the piston to the bowl if the balls weren't seating properly due to debris. That's why the bowl was still full.

2012-02-01_151911_2009-09-21_160922.png
 
You have a failed seal , casting flaw or piston failure
It is all in the master
 
Yatzee said:
The rearmost cup is not the cup that exerts pressure, rather it sucks the fluid into the piston and acts as an end seal. If any of the cups forward of this one failed ( rolled, folded back at the outer lip) you won't have fluid out the back because there's no pressure present here to cause a leak. When you pulled the piston out, the cups may have folded back to their normal position as they dragged against the piston wall. If there are check balls built into the casting, the fluid probably just moved back and forth from the piston to the bowl if the balls weren't seating properly due to debris. That's why the bowl was still full.

View attachment 446538
Click to expand...

One of these seems to be the most likely cause. I can't see the check balls if it has them, but I wondered if the seals had somehow gotten out of position when he was applying the brake. The sudden pressure loss after working great for 600 miles means something changed very quickly.

Here's what I'm worried about now. As I mentioned, I had another MC, same make and part#, that I put on. But I'm reluctant to use it now. I always considered Raybestos to be a good brand, but do I trust this other one? If this had happened at a different place and time, it could have been fatal. Is there another brand of MC I should consider? This is sort of a resto-mod, so if it doesn't look totally original, that's OK. If I have to go to a modern one, I'd rather do that than risk this happening again.
 
Dr Diff sells a 15/16 master cylinder. I'm currently using this and happy with the braking, although I have less than 100 miles since the conversion. Your experience has me worried.
 
That's scary stuff, Ive tried to teach my kids/wife not to panic but always remember the e brake in case of failure of the system.
 
Billccm said:
Dr Diff sells a 15/16 master cylinder. I'm currently using this and happy with the braking, although I have less than 100 miles since the conversion. Your experience has me worried.
Click to expand...

I don't know if I'd worry too much. This is surely a very rare occurrence. I've never had it happen before and I'm almost 60 years old. I drove some real beaters back in the 70's and 80's (always broke), and some didn't stop all that well. I had some bad MCs that produced a mushy pedal, but I don't remember one ever doing this. Maybe this was just one oddball. Or maybe a bad batch.

Thanks for the Dr Diff link. I just finished rebuilding the rear end on this car, and adding a Sure-Grip, with his stuff. Very pleased.

Mike67 said:
That's scary stuff, Ive tried to teach my kids/wife not to panic but always remember the e brake in case of failure of the system.
Click to expand...

Believe me, we're going to practice that after I get this back on the road.
 
DWinTX said:
Believe me, we're going to practice that after I get this back on the road.
Click to expand...
When I was teaching my daughter to drive, I made her practice deceleration with the automatic downshifting from D to 2 to 1 and at 10MPH slowly applying the parking brake to stop. Probably won't help in a high speed panic situation, but at least she has an idea of slowing and stopping a car without the use of a brake pedal.
 
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