I'm hoping somebody has some suggestions for me. Here's the background:

I purchased a new 914rubber master cylinder back in 2017 (doesn't seem that long ago though). It's been working flawlessly, and I only put maybe 200 km per year on the car.

I stored the car over the winter, and this spring when I pulled it out, I had zero brakes. Brake pedal would go to the floor with no resistance.

There are no leaks that I could find, and the brake fluid reservoir was still full.

I was thinking maybe the master cylinder piston got jammed open with some crud from the pedal side (the boot isn't really a tight fit), so I pulled it out and took it all apart, cleaned it and rebuilt it. Seals and everything all looked good.

Put it back in, and still the same issue.

I can always buy another master, but it seems like such a weird failure considering the dual independent circuits.

Any thoughts?

Thanks in advance!

I've been knee deep in the brakes on both of my cars over the past few weeks. NB: that doesn't make me an expert, but at least it is all fresh in my mind. If you are not losing fluid anywhere and the pedal is going all the way to the floor (and not pumping up) I can't think of anything else other than a failed master cylinder that could be causing the problem. I know I'd be replacing the MC if I was in your situation. Update everyone once you figure it out.

Keep in mind…all the master cylinder does is displace brake fluid. It will not stop displacing fluid (at which point the pedal stops moving and becomes firm) until fluid runs out of places to go. The caliper pistons should not move much (especially the fronts with the anti-knockback mechanisms in the pistons) and should not require much fluid volume at all before they can’t move anymore and the pedal gets hard. Anything that results in excessive piston movement…and it doesn’t take much to double or triple the volume requirement, will result in a low pedal. Look at wheel bearing slop, venting clearance, etc…

I typically start diagnosis of an issue like this by clamping off all the hoses (can’t do this with SS braided lines) and see what the pedal does. If it’s high and firm, your problem exists at one of the wheels…pull one clamp at a time until it goes to fooorvagain and the scrutinize that corner while someone runs the brake pedal…figure out what’s moving too much and why. If the pedal is still lousy with all the lines clamped off, I’ll then go up and dead head the master cylinder using plugs (M10x1 in this case)…if the pedal isn’t like a rock we’ve either got a bad master or it’s full of air.

The above is the way to go with one exception.

I don't clamp off brake hoses that I am expecting to return to service.
If I'm replacing anyway (20+ year old hoses are an automatic replace) I'll use the clamps but I've seen too many brake hoses fail internally to crush any I want to re-use.

The crushing often causes an issue with the inner liner which you cannot see.
Most of the time failed inner liner causes calipers to not release, and it doesn't make a leak that is obvious externally.

Now, if I'm replacing the rubber lines anyway, I'll pull brake fluid via vacuum on the open steel lines. Makes bleeding so much faster.

If you do not know the age of the rubber lines AUTOMATICALLY replace them as there is a chance they are half a century old.

Having a rubber brake line rupture is never a good day. Puppies always wait till it's really important to have brakes and then "pop goes the weasel"...

In 40 years, I've never seen a master cylinder failure that could be traced back to trash entering the cylinder from the pedal mechanism.

As far as a complete failure after 6 years, I have no idea.

Om the rebuild side, what kind of seals did you use? Organic rubber (EPDM), will be dissolved by break fluid.

Let me see if I have enough parts to make you a rebuild kit.

Mark

That would be great, thanks Mark!

I replaced one of the small o-rings on the safety circuit piston, which seemed to have a flat spot. Buna-N I believe. But as far as I can tell, failure in that circuit shouldn't affect braking function.

Excuse me?

EPDM is about the only rubber that's compatible with brake fluid which is why it's used in every brake hydraulic part that uses brake fluid.

I hope you're using EPDM in your masters.

since nobody cares what I think...Pegasus has been in the brake game for a little bit.

https://www.pegasusautoracing.com/document....DocID=TECH00147

Buna N will swell, you need EPDM.

PatMc, thanks for the Pegasus link.

That is one of the most concise articles I have read regarding the difference in rubber application.

Well worth the read!

Just stumbled on this thread.

O-rings have no place in a master cylinder on moving parts.

O-rings are commonly used as seals between the reservoir and the master cylinder or between the master cylinder and a brake pressure booster. Both of these applications are static sealing applications.

The seals on the ATE master cylinder are single lip seals design for dynamic motion. These are the same type of seals that you'll find on the master cylinder pistons, just much smaller diameter.

Single lip seal serves the purpose of increasing sealing lip pressure as the pressure behind the seal increases. An O-ring cannot duplicate this characteristic. When used in dynamic applications O-rings are prone to flat spotting due to wear as they move relative to the bore. O-rings also have substantially higher friction than a single lip seal.

A leak around the dynamic pressure differential piston seals can indeed create an internal bypass within the master cylinder. The differential pressure piston sees brake line pressure.

If the pressure is balanced between the two brake circuits (front & rear), the differential pressure piston doesn't move. If one side or the other is a different pressure, the piston will move and triggers the brake light switch.

Single lip seals on the pressure differential pistons in ATE master cylinder:
Click to view attachment
Click to view attachment

Tell that to Bosch....this master cylinder (a variation of which is installed in just about every FedEx/UPS truck out there) has nothing but o-rings...no lip seals.

Not a Porsche, but they last just fine.

Always willing to learn something new and have opportunity to eat crow. Pictures of internals and where the O-rings are? Not finding on Google
@PatMc

I don't have a picture of the main seals...it's been a few years since I was into one...but I have torn apart many (I used to do all the warranty inspection / failure analysis for a brake parts manufacturer) and they are all o-rings. Here's a picture (screenshot of a video) I made showing the DPV in a cutaway I did trying to explain to UPS that it's supposed to work like this and the brake light on after install and bleeding does not mean its a bad master cylinder...just means need to recenter valve (and read the Bosch literature that says the same thing)...

Note the o-ring on the shuttle valve. I know, not what you asked for, but the best I can do on short notice.

Thank you - I've learned something.

I can see how the O-ring might be a cost saving measure both for cost of the part + easier assembly. Likewise I can appreciate how the DPV could be considered a "static" part given if both circuits are operating properly, the piston of the DPV never moves.

I am curious though about the main piston seals still

A little digging and it looks like that master cylinder you're showing is from the Ford F59 heavy duty chassis and is probably using Hydroboost where the power steering system is being used to augment pedal pressure instead of a conventional vacuum booster.

So maybe O-rings in application where pedal feel doesn't matter much and can tolerate the friction and wear since the Hydro booster is doing the work of building line pressure rather than the driver's leg?

Overall, I stand corrected on the DPV based on your pictures - Thank you for enlightening me

How many other suppliers or OEM's are you seeing O-rings besides this application? Are they all in heavy duty industrial chassis applications?

The piston spring on these is likely larger than the ones holding up a 914 lol.

This is the only application where I've seen exclusive use of o-rings. I typically share your opinion of them in dynamic applications, but my point was that they are used successfully. Airheart brake calipers (old race car stuff) used o-rings instead of square cut seals. It's tough to make them not leak these days, but they evidently worked OK at some point otherwise people wouldn't send them to me to rebuild. I try and avoid them at this point.

Over 20 years in the brake business and I still learn new things every day.

I meant to comment on your theory of brake hose damage with clamping.

My experience has shown that if the brake hose is in such a condition such that clamping them off (with line locks, not vise grips clamped down with both hands) will cause damage to the hose, then it should have been replaced anyway. All brake hoses start out straight....if you can't stretch it perfectly straight...it's a good idea to replace it. Hoses are cheap, the problems they cause when they fail are not cheap. Any hose less than 10 years old should be able to get clamped off without any problems. Older ones, who cares if they get damaged...they're getting replaced with new ones as soon as the diagnosis is complete.

Only difference between me and you is our limits on age.
I'm not replacing brake hoses at 10 years. I'm replacing hoses at 20 but not pinching them at any point in their life.
I do agree that when I find one that has been damaged by some monkey pinching it off you can always see the teeth marks from the vise grips on the hose...

Most of the folks on a forum like this do not have the special pinch off clamps that you and I have and I try to include the expected users tool capabilities in my posts.
You said to "pinch off all of the hoses and test system."
Even I only have two of the clamps and I own 30,000 lbs of tools.

You didn't say to make sure and use the proper tools in that first post. Without the warning vise grips will end up getting used.
Needle nose vice grips can be used for this operation IF you cut a piece of 3/8 fuel hose, slit it lengthwise and slide it over each of the jaws. You can make a pair of "soft" clamps via this method.

I've not had to pinch a brake hose to diagnose a system ever.
My wife's 06 Odyssey, (17 years old/210K) is still running its original hoses. I've flushed the fluid at reasonable intervals, never pinched the hoses, and will not replace them before I replace the vehicle in the spring.

Update: I put the master cylinder back in, bled the brakes and now everything seems fine. Is it possible that an air bubble somehow developed over last winter, and that was the problem all along?