Hi everyone.

1978 F350 CC Custom 351 Modified Auto Trans 3.73 Dana (not sure but think it's the 10.25)

My brake pedal was almost to the floor. I replaced the master cylinder (leaking out the rear) and the vacuum brake booster (it may have still been working but hey, it was getting really old), replaced the rear shoes, adjusted the rear shoes all the way out and backed them off just enough to where the wheels turn with a little drag. The wheel cylinders appear to be fine. The brake drums are within tolerance. The problem is that when I step on the brakes the fluid leaks out through the top cover of the master cylinder. With the cover off I notice a lot of bubbling or fluid movement in the larger part of the reservoir. I bled all four brake lines three times each starting with the wheel furthest away from the master cylinder. It feels just a little soft, but not really spongy. The rear wheels will lock if I stomp on the brakes. I removed the brake lines and plugged the master cylinder and the pedal was rock solid (and, of course, bled the lines again). I bled the master cylinder before installing it. I adjusted the pushrod from the brake booster to what seems like the right amount of play. Anyone have any suggestions or a similar experience? I'm all out of ideas.

 

Put proportioning Valve in the search bar. I remember reading something about having to pull a pin out in order to properly bleed the front brakes.

 

One of the problems I have noticed is that in the past few years, most of the replacement master cylinders are not machined where the lid seals.

This can lead to seepage and air and moisture contamination of the brake fluid.

The last one I bought was so rough I clamped it on it's side to the edge of the work bench (so it would not get filings in it) and filed it smooth so that the lid would seal.

This could or could not be your problem but something to check.

 

The fluid shouldn't "leak" from the cover.

Does the sealing surface looked machined or have irregular imperfections?

Did you bench bled the master before installing?
.

 

I have to make sure the gasket and the top of the reservoir are perfectly clean any time I install the cover after checking the fluid or I have the same problem. It actually drained the rear section once before I realized it was leaking. As long as it's clean, though, seals fine and I don't have a problem.

BTW, you were smart to replace the booster if the MC was leaking out the rear. There's a good chance that some fluid ended up in the booster which eats away at the diaphragm. You'd probably be tearing it apart again soon if you hadn't already replaced it.

 

Is your master cylinder cover vented? there's a small vent punched into the original MC covers, most replacement MC's (both reman'd and new) do not come with a vented cover, though some do.

I went through a similar scenario you're describing when i replaced my MC two or three summers ago (with a new MC, not a reman'd) brake fluid kept leaking past the MC cover gasket.

It had me stumped for awhile, the sealing surface was smooth, the bailing wire was tight, but yet it still kept leaking... so on a whim i replaced the non-vented cover that came with the new MC with my old vented cover, problem solved!

The MC has been leak free for over two years now, this may or may not be your issue but i thought i'd relay my own experience, hopefully it helps.

 

Thanks everyone for the great suggestions. Yes, I bench bled the MC before installing, held the pin out on the proportioning valve while bleeding the front (and the valve appears to be functioning correctly). The "new" rebuilt master cylinder does not appear to be smooth on the top, mating surface just like some of you described. I cleaned off as much fluid as I could and put some black high-temp silicone on the top, gasket side and it still leaks. Next I will try using the old lid! I thought about it but couldn't think of a good reason to try it. A vent is a pretty good reason! Since the front, larger reservoir is for the front brakes and that's where the problem seems to be I'll try bleeding them again with the proportioning valve pin held out.

The good news is that, other than needing new U-joints, that's about all that's wrong with the truck (other than being old and ugly--but so am I).

 

Mine did the same thing after a rebuilt MC, I guess I got lucky because I just repositioned the gasket several times and it quit. Next time I check the level it will probably leak again lol

 

im having the same issues on my 78 f150 after i installed a new m/c. where is the breather hole located on the cap? i might just have to drill a small hole in the new cap

 

Circled in red... there's a 'channel' stamped into the cover that connects the front and rear reservoir which allows the rear to vent.

 

Is it just a hole or is there a check valve?

 

Ha! Look at this.

Buy Spectre Master Cylinder Cover 2-3/4" X 5-3/4" Ford Single Bail 4224 at Advance Auto Parts

 

It's just a simple hole i.e. vent.

 

Drilled one, no difference. If you think about it, how can it vent to the reservoir? The "gasket" is a solid piece of rubber which should seal each reservoir.

 

You're missing the point i referred to in your previous thread.

The gasket i.e. collapsible diaphragm acts as a "bellows", it moves up and down, in other words, it expands and contracts in response to the pressure within the reservoir, hence the vent, without the vent there would be a vacuum lock and the fluid would be forced out at the gasket (unless you only fill your reservoirs half full).

If this isn't your issue than i would suspect, as others have said, a rough casting not giving you a smooth sealing surface between the master cylinder and lid.