I have 3 different sizes of brake MC - 0.7 , 0.75 and 0.625. The smallest one has two rubber seals on the piston ( although it is the same model / manufacture as the 0.75 one - Wilwood (regular one, not compact) ).
All are used, real usage was short, but the car was standing quite long not running. Fluid in.
The bigger two have immediate return after pushing the rod / piston inside - piston returns immediately.
The 0.625 - it takes longer , maybe not 5 seconds but 0,5-1-1,5 seconds. If I take the smaller (additonal) oring off - the return is immediate as on the bigger master cylinders.



My question is (as I never had in hands other than those few MC) - is the construction- two orings/rubber seals typical for smaller MC in contrast to one seal for biger ones ?

Is for the two - orings type of MC "normal" that the return of the piston and rod is slower ?



I have cleaned all parts (someone used regular - garden type or sth oring under the hose for brake fluid reservoir , and it solved in gel like substance , thought that maybe this caused this slow return, but not, after cleaning interior, piston etc - little better, but not much) , dryed, appiled some brake fluid before tests. Tested on the bench.

Here is how the seals/orings looks (bottom - 0.625) , this upper one is from other , old MC, dont look at rust, just wanted to ilustrate the seals difference


https://drive.google.com/file/d/14n7nx5rC3HtgGtSOpED-egfviFZXSo2q/view?usp=sharing

Hi,

The important seal is the long, conical shaped one. The other tore-shaped o-ring at the end is certainly there just as a dust-seal only.

If it has been replaced by a non-EPDM o-ring (Nitrile, or other kind of rubber), it might has swollen in time because the rubber is not compatible with brake fluid, hence the resistance when the piston returns.

First, make sure you clean your parts and master cylinder with the right products. Petrol based cleaners (thinner, white spirit, petrol, etc.) are no good for brake fluid which does not dissolve in them. There are brake fluid cleaner sold everywhere, but the best and the cheapest is domestic alcohol (house cleaning, etc.)

Regarding your o-ring, the cheap thing would be to change this or-ring only, making sure you replace it with an EPDM one (brake fluid compatible rubber). Outside diam. should be very close to your piston diam.

The best thing would be to buy a complete repair kit for your master-cylinder. You should find some on e-bay.

Hope that will help

Thank You!

in fact I was using as You described - petrol based liquids to clean. Will try with proper one.
As for the material of o-ring it is propably the proper one for brakre fluid, no sighs of solution and I think it is the factory one (car is rarther fresh) .
I know what can brake fluid do with rubber - someone fitted the adaptors for remote brake fluid reservoir on those master cylinders - sealed with rubber seal - solved in a gel-like substance, covering part of the thread.

Odd they are using an O ring as a dynamic seal, as you have discovered it will creat friction and if its the wrong material it would have swollen a lot and probobly jammed the whole thing up. Years ago at great expense I bought a Willwood pull cylinder for a clutch conversion, after a short time the pull rod came loose from the piston resulting in a tow home. The rod was simply screwed into the piston with no locknut or locktite. They just used an O ring as the main seal which will always weep slightly instead of a lip seal, at least when I complained to them they sent a new seal kit but it seemed poorly designed and made for what it cost. Sorry for the rant.

The main seal (this one in cone-like shape) is there, the Oring looks like an additional one.

I ended up with OBP master cylinder - immediate return. Did not dissasembled it, so dont know if OBP is using Oring also.

Other little difference I have found - on Willwood there is more play on the longitudinal movement than in OBP master cylinders I had.