Loss of pressure in rear brakes

[RottenRawb]

I have a empi front and rear disk brake system with a dual circuit master cylinder. I have been driving this system for 7 months with the following issue only happening once four months ago and it corrected itself some how. The issue then happened again yesterday and is as follows:

I was driving to work, brakes had been working fine for about 12 minutes of driving/stopping, not alot as I remember it was 430 in the morning. I was cruising towards a left turn and stepped on my brakes and they went almost to the floor. As I came to a stop it seemed as though the rear right tire locked up, well a tire locked up and thats where I heard it. I continued driving to work with a very soft pedal. I then had to drive three time since, using downshifting and braking early but obviously need to get this fixed.

I have checked all of my connections, several times, no leaks. Fill tank is full, like a millimeter off top. I tried bleeding and am getting ready to try again. Both front spit out clear fluid with no bubbles. Rears, however, let nothing out, save for maybe one side showed a slow drip.

So I also watched for brake lights coming on, my rear lights were only hooked up to the circuit closest to the firewall and when I depressed the brake pedal, the lights flick on and immediately off were they stay unless I pump the pedal. As a test I reconnected the brake lights power to the outer switch and now when the brakes are depressed, I have full functioning lights.

Curious if anyone has any trouble shooting tips left or if they know where my failure lies.

[Piledriver]

95%+ probability the master cylinder needs new seals.

I actually can't think of anything else that could cause a self correcting brake issue like that, so I'm tempted to say 100%.
(I'd just go find a decent new one, or "lifetime" unit from a FLAPS)

[Ol'fogasaurus]

Do you have residual pressure valves installed?

Lee

[RottenRawb]

No, should I? I thought it was enough to run the dual circuit? Where do I run them?

[Ol'fogasaurus]

Residual pressure valves are used when the slave cylinders sit higher than the master cylinder (MC) for a period of time; the idea of the valve it to stop "bleed back" which is the brake fluid, over time, is running back into the MC (our dear friend gravity at work). When the fluid drains back into the reservoir the brake shoes retract/pull-a-way” from the drums than you would normally have which leads to no brakes or pumping the brakes to get line pressure back. The residual pressure valves keeps fluid/line pressure which in turn holds the shoes/pucks close to the drums/rotors.

The generally accepted rule of thumb is 2# valves for disc brakes and 10# valves for drum brakes although I know some off-road guys who like to have 10#s on discs. If you look at some brake diagrams they are put in line by each slave (wheel/caliper) cylinder but I am not sure that is always necessary. I would think that one on each outlet line of the MC might be OK but I am sure you are going to get some push back from others on that idea.

Not all MCs are the same: I hear that some MCs have residual pressure valves built into them; some cars have the valves in the brass distributor block for the brakes. I have heard that some VWs dual chamber MCs may have valves in them but someone else would have to tell you which ones if that is true.

Lee

[Piledriver]

The later MC have tiny drillings to get a similar effect, no residual valves.

I still bet the seals are toast, I have seen new MC that have seals that have taken a set just from sitting on the shelf.

Marc psted a great explanation awhile back, basically if you STOMP it and it works, but if you ease into the brakes and they go to the floor, it's almost certainly the MC.

(the pressure spike of hard braking expands the seals by design)

[Ol'fogasaurus]

“I have checked all of my connections, several times, no leaks. Fill tank is full, like a millimeter off top. I tried bleeding and am getting ready to try again. Both front spit out clear fluid with no bubbles. Rears, however, let nothing out, save for maybe one side showed a slow drip.”

This is what bothers me the most: as I understand what you are saying the rears do not bleed which could translate to the rears not working. Do an inspection of the soft lines to see what condition they are in as they can be bad from the factory. Look for kinks in the hard line also. If everything looks OK then start disconnecting lines and check for fluid being able to pass; I think I would start at the rear connections to the brakes first (the easiest ones to get to) then move to the distribution valve at the rear of the tunnel where the brake lines are split since it is both rears that do not want to bleed. It is possible that there is something plugging the system here.

I have had bad MCs "off the shelf" myself but the "valves" are another option to look at/into as well as a combination of the two problems. I don’t remember of any way to check the MC for seals gone other than just replacing it. I would do the line check first before springing for a new MC and finding out it was not the problem. It could be that the one chamber going to the rear was bad too; bench bleeding that may have shown a problem if there was one.

Lee

[RottenRawb]

If I pump the brakes with the bleeders open I can get fluid from both rears. Again, there are stainless steel lines running to all but the front right wheel when it was replaced after an accident. Already bought the master, just trying to figure out if I need to get the valves before tearing into it.

[Marc]

Make sure there's freeplay on the pushrod when the pedal's up (spec is 1mm, but anything perceptible is enough). If not, the rear piston can't come all the way back in the bore and uncover the compensating port, so no fluid gets in from the reservoir.

[RottenRawb]

Changed the MC and brakes back to normal...apocalypse averted. Thanks for the help.

[Ol'fogasaurus]

Good to hear it was a simple fix.

Lee

[Piledriver]

Thanks for following up: we all learn something.