I bled the brakes after an autox about 2 weeks ago. got some air out of the rear and the pedal was feeling great.

Today I was sitting in the car in the driveway waiting for it to warm up and all of a sudden the pedal moves to the floor and the brake warning light comes on.

Now there is almost no pedal pressure and it doesn't seem to pump up even.

Is this a total failure of the master cyl or does it act funny once the brake warning light is tripped?

It is a OE cylinder from PMB about 7or more years ago but not a lot of time on the car.

Do you still have fluid in the system? If fluid has remained in the system I'd say that MC is toast.

Yes there is still fluid in the system

Get under the car and see if the MC is wet.

I take a look at it tomorrow
Thanks

Cylinder is dry but doesn't build pressure.

Does anyone know
Do the master cylinders that 914 rubber is selling have the high quality seals like the OE ATE ones PMB sells?

When you bled the brakes are you certain that you fully seated the bleed valves? Check all four corners for wetness. If they are all dry, then most likely your M/C is toast. When you use the two-person pump and bleed technique, you can get the M/C piston moving to the far end of its range, resulting in the seal running through corrosion at the far end where the piston normally does not go. This will tear up the seal and causes this kind of failure.

I used a pressure bleeder so that shouldn't have done it. I'll check the bled screws when I get back home.

Anyone have any experience with 914 rubber's master cylinders?

I'm running 2 of their MC's. So far so good! And yes, your MC is toast!

Those seals typically don't have catastrophic failures. If you used a pressure bleeding method to get air out of the system you should not have damaged the internal seals. Its when the pedal is fully extended that seals can be damaged.


I would be curious of tge mode of failure.

I installed a 914 rubber MC in my BB along with PMB calipers and a pressure regulator. It took me about three months and a gallon of fluid to get the system to where I am comfortable driving the way I like to drive. I seem to remember that the MC is an ATE that 914 has fitted their feed line grommets to and adds the new feed lines. I had no problems with the install or bleeding the MC. I would by another if I ever need a new master.
Rob

Rob
How long ago did you install that cylinder?

I've been running a 914Rubber MC for 3,000 miles now and it has worked well. I had an issue that AFAIK was unique to a few MCs produced in their first run of MCs and was easily fixed. They are NOT ATE MCs with custom fittings added. I'd still rather have a 17mm MC though.

I replaced the failing original 17 mc on my 71 with a stock ate 17mm. I prefer the slightly spongy, easy to modulate feel over the firmer feel of a 19mm. I have used both on many different cars. I’m sort of picky about this and many people don’t care but it’s been the same on all my road and mountain bikes. A softer progressive feel compared to a harder, on-off switch.
It’s more of an issue on a mountain bike where your hands get worn out and loose strength over a long ride or race but there’s no getting around the physics that a smaller mc will provide higher pressure to the pedal caliper (at the small expense of increased pedal travel).

I have had crud in the braking system destroy a master cylinder before as I was flushing it. The fluid was pretty old when I did it. I wasn't driving the car much and Covid didn't help. I overlooked flushing for a few years.

Good thing the reservoir covers will be ready soon.

So turn s out it probably isn't the master cylinder.

i rolled the car out of the garage and found a puddle back by the engine.


After pressure starts to build in the system brake fluid squirts out where the red arrow is pointing. Between the casting and the metal bracket.

Anyone experience this issue before?
Click to view attachment

PMB sells a rebuild kit, or you can send it to them to be rebuilt.

Thanks

As much of a disappointment something like this is…it may be proof you have someone watching over you. So glad this didn’t happen on a twisty canyon road.

jmargush: I started putting the system back together around April, finally got it to the point I was comfortable in September. I don't feel that the MC was an issue. Mostly air in the system and problems with the rebuilds that I received from the vendor. Several back and forth's and with shipping time it all added up.
I seem to be good now although the restored calipers are already starting to show signs of corrosion through the anodizing. Even thought they have yet to see water, or 500 miles.
Rob

Camping onto this discussion, as I don't want to open a new thread...

Re: the brake master cylinder switch. From diagrams and the service manual I see that it's a simply shuttle valve, ported between the front and rear chambers. If one port has low(er) pressure then the shuttle valve moves over one direction and engages the pin that grounds the switch. It seems to have spring loading on either side to auto-center the switch when the differnetial pressures is zero. All correct?

From the service manual diagram, it also appears that there's o-rings on each side of the shuttle valve, implying that there should not be any brake fluid in that central chamber where the pin/switch is. Still correct?

So here's my issue. I installed a new 19mm master from Pelican today, and I'm getting weeping out of the switch. I thought maybe it was a bad switch so I replaced it with the old one but now I see the little vent in the center so I'm guessing there should not be any brake fluid in that central chamber.

Accurate so far? If so, then it seems I may have bad o-rings (or a bad bore) in that shuttle valve.

I'm not seeing weeping/leakage anywhere else, and this is on my race car which will never have the light functional anyway. Any pressing reason why I cannot just replace the switch with a bolt and not worry about it? I'd prefer to not have to replace the master again, though I will notify the vendor of the issue.

Thoughts?

GA

It wouldn't bother me to replace the switch with a bolt on a race car.
but a new MC shouldn't leak anywhere. What does that say about the future of the other seals?

Right.

I've been mulling this over the last few days and I think I'm going to swap the old one back in and return this one (and maybe the second one I bought) back to Pelican.

- Turns out my old one was working fine and is already a 19mm. I didn't realize that and ASSumed it was the stock 17mm because putting in a 19mm in this race car was non-compliant to the regs. Shame on you, JP!

- It bothers me that if this is a slow weep while sitting on the lift, it may be a stream when under pressure. If that is the case then I'd now have a single-chamber master cylinder (pressure could bypass through the shuttle valve bore) which would not make me very comfy as I'm barrelling into Turn 1 at 120mph.

- And as you noted, if this simple bit has a problem, what does it say about the rest of the part?

That is a defective master cylinder. More likely a seal got rolled or nicked during assembly....it's rare, but it happens. Exchange it for a replacement

Follow up on this.

I replaced the master with another Pelican-sourced URO I have (I ordered two 19mm as I have two cars) and the second one is not leaking. Pelican sent me a prepaid label for return/replacement.

Just as aheads-up though, there's a guy on Facebook that had a problem a new MC SPEWING fluid out of the vent hole when he pushes on the brake pedal. I mean, his video shows it squirting out that hole across the width of the car out the left side, as if there's no seal in the secondary chamber at all.

Mine is labeled a URO, his is labeled "FTE". I suggest we need to keep our eyes open on these replacement MCs...

Dude,
I can hook you up if you need a new master cylinder. You know where to find me.