I converted my 68 Dart over to front disc brakes from a 1973 Duster system. The problem is when I pump the brakes fluid dumps out from under the cover. Leaks right under the rubber gasket. Fluid level about about 1/4" from top of reservoir. This is the second new master cylinder I put on and both did the same thing. Do I have a bad proportion valve? Never had this happen before, did this conversion many times. Brakes work fine. Lines all put on correctly, checked twice. Checking to see if this happened to anyone else.

i had this messy problem also with 2 rebuilt masters tried different covers that didnt help so i just purchased a NEW master and the issue was gone.i assume that maybe the top of the master cylinders maybe had imperfection in them from being dropped into core collecting barrels too many times.

did you look closely at the MC top sealing surface for any imperfections that could cause the gasket to leak ? check the gasket closely for tears ? look the top over for being straight ? loose, or wrong bail wire for lid ? just throwing stuff out there. i really don't think prop valve has anything to do with this.

Might be able to bend the cover wire to tighten the lid. I have had them loose enough to where you could slide the lid around...

If needed it isn't hard to sand that sealing surface flat.

You guys had to give my brain a kick start. I went out with a 3x5 block and some sandpaper. The top was flat but it had the cutter marks in the top surface. I sanded the top until all cutter grooves were gone. Put the top back on, pumped the heck out of the brakes and no more leak. Thanks for all your input.

Excellent

glad you got it "covered" [pun intended ]

I'm glad somebody is in a good mood today

what's going on today RR ? i'm just wore out from building a field stone wall the last couple of days. only have about 12 more rocks to cut and position. don't know if i can muster up the energy for that today or not.

rest up for a day (you deserve it). Rome wasn't built in a day & mistakes can happen when we are tired/preoccupied. I'm gonna lay in bed & watch football all day.

How did you keep the metal dust out of the brake fluid?

Looks like a majority of the dust stuck to the sandpaper. No visual dust in the fluid. Skimmed the top of the fluid with paper towel, no dust apparent. Shouldn't effect anything, I've seen heavy rust in old master cylinders in the past and they worked fine.

I am wondering why pumping the brakes makes fluid level rise in the MC. That is not suppose to happen. Once the compensating ports close it is a sealed system otherwise no pressure builds up. In fact the gasket/lid has an opening to atmosphere. Sure both front and back are working?

It doesn't make the level rise. What happens is that until the fill ports are closes it will squirt a bit of fluid back into the reservoir. The faster/harder you hit the pedal the higher it will splash. If you ever bled brakes by pumping the pedal you seen it.