My brake fluid is draining from the small reservoir and overfilling the rear reservoir,after driving it ..Its the 2 or 3 rd one doing it,,,proportioning valve bad??????thanks in advance.

Sounds like its bypassing internally and that happens when the master is bad

Thanks for the advice STUMPY,I guess since I am not on here much anymore,no one will chime in....

No this is a very unusal problem, I've been fixing cars for over 30 yrs and never seen this. No idea why it might happen good thing brake parts are generally cheap.

I'm experiencing this problem on my 92 pickup too. The original master did it and a new, not rebuild, master is doing it too. My 77 pickup with the same master does not do it.

Trucks have identical brakes except the 77 has a adjustable p valve. Im suspicious that the 92 p valve is allowing the bypass.

Im thinking of eliminating the p valve on the 92.

If you have power brakes I would check that the output shaft from the booster isn't adjusted slightly too long which isn't allowing the MC piston to travel completely back to zero. When that happens the MC does crazy things. There should be a VERY small amount of dead travel before the booster pushes the piston

It would help if you identified the car/brake system. Old type, vice a modern car with antilock brakes etc... Not enough info to even start helping you.