Brake master cylinder valve, help!!

[david4451]

On the master cylinder, there is a "valve" that feeds The rear wheels and the brake line to the rear screws into it.
On this "valve" is a groove with an thin '0' ring on it, under the '0' ring is a pin hole and this pin hole is leaking brake fluid, only when the brake peddle is pressed.
Could someone please help me with why it is leaking like this, thanks in advance.

UPDATE: Pleased to say that I've fixed this leak, disassembled the "valve" and cleaned out deposits.

 

[david4451]

Another UPDATE !!!
After cleaning out the valve on master cylinder (which no one seems to know about) it started leaking again after a week or two, which was a PITA.
One problem is that the break fluid is making it to the chassis, this is causing the paint to bubble and rust.
I'm sure that many ST owners must also have this problem and may not know why.

The fix was putting two new wheel cylinders on drum brakes at the back. My old wheel cylinders were not stuck solid but the pistons were not moving freely.
My assessment is that there was a back pressure to the VALVE on the master cylinder which caused a small amount of brake fluid to leak from the VALVE pin hole, under the "O" ring.
I have a container which I put under this master cylinder VALVE to catch brake fluid and since new wheel cylinders, it is bone dry. The other bonus is no more paint bubbling on chassis.

 

[Fredness]

1. Part of the problem may be due to WHERE you posted your request. Brakes aren't "2001-2005 Explorer Sport Trac" specific or unique, they are platform wide. Posting your question in the a larger audience in the "Stock 1995 - 2001 Explorers" (what our 2001-2005 Explorer Sport Tracs are based off of) may have gotten a better response.

2. Pictures help a LOT. Since I've never seen a "valve on master cylinder", it may be called something else, or been added by a previous owner. A ROUGH guess is that you are talking about the Brake Pressure Differential Switch/Proportioning Valve.

3. Finally, there should be no leaks or drips from the brake system ever. The overly simplified answer is to replace the part that is leaking...

 

[david4451]

Fredness thanks for reply, next time I'll post forum wide.
After the original post I did find on Google the description of a "proportioning valve" but no images of the one on the ST, so I assume it is a proportioning valve.
I agree there should never be any leaks in the braking system and that was why I posted in the first place. However it looks to me to be designed to leak! Why would they put a "O" ring over the pin hole(put there by manufacturer) in the valve?
By the way the valve is hexagonal, aluminium and maybe 1.5 inches long, comming out of master cylinder with rear brake line attaching to it. This part I could not find sold anywhere and I'm a Brit living in Philippines and if I wanted a new master cylinder Ford want $500!!!!
As it has turned out the new wheel cylinders fixed the problem which exactly why I'm not sure.