Master Cylinder Rust/Leaks

I'm sure all of you have dealt with this at one point or another if you have a stock or aftermarket master cylinder that's not one of those fancy chrome pieces.

What I am talking about is it seems like as soon as you take a replacement out of the box, the humidity in the air oxidizes and the cast iron changes from fresh gray to rust brown. I've tried to combat it with clear coat and my father ended up painting his silver. Clearing it helped (I did it with Krylon rattle can vs actual gun clear though, which probably did not help), but in the end it still rusted. What the heck gives? It's one of my biggest peeves.

Also, is it just me or have aftermarket cylinders gone to crap? I cannot for the life of me find a factory master, new or reman, that doesn't leak fluid into the brake booster. Every single one I have has leaked within 6 months, even from just sitting. Steering and brakes are one thing I don't mess around with and any leak is immediately resolved, but this one is kicking my butt. I should not have to replace the master cylinder twice a year or more even. This being said, they do not fail and I have never lost my brakes. They just weep down my booster.

I must stress, these are not always Autozone special parts store master cylinders. I have bought quality ones, cheap ones, remanned expensive and cheap...all to no avail. Not only does it become expensive, but I work 2 jobs and I don't really have the time to bleed them out by myself every time. Partially the reason why my 79 has sat for a year and a half. Any help?

Thanks guys.

I can't help you with the leaking part, but the picture below shows what I did to get rid of my rusty master cylinder. I installed a new aftermarket MC for a 1987-1991 truck. I also did away with the prop valve on the frame, and ran those wires to the float in the new master, so my brake light will still illuminate if my fluid level drops. I stayed with the Bullnose booster, although I did replace it.

Eastwood Restoration mag wheel clear coat works fine. I sprayed mine when I got it from Rockauto. Take it from box, wipe it down with acetone, put 3 or 4 coats of that clear on it. Worked for me.

Rembrant wrote

I..... show what I did to get rid of my rusty master cylinder. I installed a new aftermarket MC for a 1987-1991 truck....

I was going to suggest.  

ArdWrknTrk wrote

Rembrant wrote

I..... show what I did to get rid of my rusty master cylinder. I installed a new aftermarket MC for a 1987-1991 truck....

I was going to suggest.  

Yep, that is going to be my approach as well.

Hmm, I didnt realize it was that easy. Although I might still take the clear approach just for period correctness.

Thanks gents.

Also not dealing with leaks but with the cosmetic approach -

While the truck was in the body shop I picked up a new MC and booster. The MC I disassembled and powder coated. The booster I painted with Eastwoods brake booster paint. Both look really good right now but neither have seen any usage so can't speak to longevity or how they hold up to any fluid that may get on them be it short or long.

Angelo Voltura wrote

Hmm, I didnt realize it was that easy. Although I might still take the clear approach just for period correctness.

Thanks gents.

If switching to the later aluminum and plastic master cylinder, there are two important things to keep in mind. The little valve I'm pointing to in the picture below is not included with a new master cylinder. The new master cylinder does come with a new o-ring for it, but that's it. I went to the junkyard and grabbed two of them, one to install, and one for a spare. I think they were $5. The other important thing is that the front and rear brake ports are reversed from the old cast iron master cylinder.

I'll have to apologize, because I do not know what that little valve is called. Maybe Jim knows?



I get the period correctness part as that is something that is always on my mind when considering parts and/or modifications. I struggled with that as there are certain things I prefer to leave original if at all possible.

I recognize that valve, I've sold a few as a parts winger over the years.

I'll keep it in mind. I'd imagine it probably would fit my 79 as well as the 85 since they are the same master.

Other than swapping the brake lines around what else is involved
in swapping to the newer master cylinder?

I'm pretty sure that's the residual pressure valve for the rear brakes.

There's really nothing else to the swap.
You have to be sure to get the the correct bore M/C.
150 & Bronco or 250/350.

The bores are different.
And the C-C of the mounting studs on the booster has to match.the base flange.

Do you to do away with the original proportioning valve?
Does the residual pressure valve do the same thing?

This has been discussed multiple times here on the forum.

It is considered by Ford a 'combination valve'.
Shows failure of the brake system and the internal shuttle should shut off flow to a leaking front or rear circuit.

The later master cylinders -once RABS came about- have a float to show they are running low.

Frogger2662 wrote

Do you to do away with the original proportioning valve?

I did remove it, yes. I ran all new brake lines on the truck, so it was a convenient time to swap to the later style master cylinder. I took the two wires from the old prop valve and ran them to the float in the new master cylinder, so I still have a functioning brake warning light for a leak (well, low fluid really, for whatever reason). It really is a very easy swap. Of course it is not period correct, but there are some benefits...mainly for me that it wouldn't rust, but it is also nice to be able to do a visual brake fluid level check when under the hood.

I believe I re-used the old fittings (from the old master) as well. There was one of them that I was having trouble finding locally.

ArdWrknTrk wrote

I'm pretty sure that's the residual pressure valve for the rear brakes.

Right. I couldn't remember what it was called. It is a definite requirement when installing the later master cylinder as you cannot bypass it. It seals to the master with an o-ring, and has a port to accept a standard flared brake line fitting.

The good news is that they are easy to find in the junkyard, and very easy to remove. I cleaned mine up on the wire wheel and it looks like new. I even grabbed a spare, although I'll likely never need it.