Brake fluid moves from front reservoir to rear

[Junkie]

'73 C20 Camper Special

yesterday the brake warning light came on while I was driving. I checked the fluid level, the rear was WAY over full (it spilled some when I removed the cap) and the front was low. I added some fluid to the front. Light stayed on. On my way home, the feel changed: I now have a very soft pedal for a couple inches, after which it firms up and the brakes work well.

Any suggestions on what to look at? Bad seal on the master somewhere? If so, suggestions on brand to go with (or avoid)?

 

[1987 GMC Jimmy]

Are there any leaks happening, particularly wheel cylinders, that could be causing a loss of fluid and therefore hydraulic pressure? What do the brake hoses look like? An internally collapsed hose, more specifically the rear one, could also be problematic by restricting the return flow of fluid. However, you’d probably have increasingly sticky rear brakes if this was an issue. Finally, I would watch out for the master cylinder like you’re thinking. The wonky brake feel makes me think that’s a likely culprit. I have a new Cardone master on my Jimmy and a new Raybestos master on my Caprice. Both work well, but steer clear of anything remanufactured. I checked the prices on RockAuto. I’d go for the AC Delco if it were me. It’s cheaper than the Raybestos and only a few dollars more than the Cardone, which some would be hesitant to put on their vehicle anyway. I don’t want to rule out a proportioning valve issue; those are virgin territory for me. Maybe someone else will follow up.

 

[highdesertrange]

bad master cylinder. highdesertranger