Non-Mustang Brakes (Sorry)

[73inNH]

Sorry in advance, but this forum is too helpful to go anywhere else.

1993 F250 4WD.

A week or so ago one of the read brake lines rusted through. Fluid poured out, no brakes. etc.
I add a new line installed and all was well.

Today, a few days after getting it back, I pushed the pedal real hard at a stop. After a second or two it dropped to the floor all of sudden! Luckily I was close to home and could make it back with ebrake.

So I was expecting to come home and see the master cylinder out of fluid. But it's not, it's full. And I don't see a leak under the truck. When I push the pedal, it drops to the floor and if I hold it there hard, there's a little braking action, but minimal.

So, since it doesn't appear to be a leak, what's my next most obvious "It must be this" repair?

 

[c9zx]

My guess is the master cylinder is bypassing fluid internally. If it is, it is time to replace it. Chuck

 

[detritusmaximus]

ABS on it?

 

[73inNH]

ABS on it?

I believe yes, rear.

 

[72XR007]

I have worked on that same setup with lousy results. I should have replaced the ABS with a new unit but the only sure way I could get it to work was eliminated the unit all together to get my pedal back.

 

[73inNH]

The ABS unit was the issue? Not the master cylinder?

 

[mrgmhale]

Pretty much any time I see brakes go to the floor, intermittently at first, and no external leaks I find it is the Master Cylinder that is leaking internally. You can rebuild, or replace new/rebuilt, depending on your level of experience. One thing to look out for, however... On Master Cylinders being use4d with disc brakes, there is NO Residual Pressure Check Valve (RPCV) needed (or wanted) for the output port feeding the disc brake calipers. The RPCVs are only for drum brakes. If you use them with a disc brake hydraulic line the disc pads will be dragging on the rotors continually, instead of releasing with the brake pedal off. That leads to loss of vehicle performance, lower MPG, over heated pads and rotor, rotor warpage, and excessive wear.

On stock Master Cylinders the RPCV is a small. black "flapper valve" under the brass seat of the hydraulic outlet port(s). If you have such a black valve under the brass seat for the disc brake calipers you need to either get a different Master Cy;inder made for front disc brakes, or remove the brass seat, take out the black, rubber RPCV and the small spring under it, and install a new brass seat.

Some folks have installed an inline RPCV as well, whether it was needed or not. The inline RPCV is just as bad for use with a disc brake hydraulic line as the ones that are installed in the Master Cylinder.

That said, you always want to use a RPCV with the hydraulic line feeding drum brake wheel cylinders. The residual pressure is what keeps air from being pulled into the wheel cylinders when hot brake fluid cools off. Any such intrusion by air gives you compressible gas in the drum brake hydraulics, which is a bad thing. The pressure on the wheel cylinders helps keep the cup seals inside the wheel cylinders pressed tightly against the inside of the wheel cylinder bores. A secondary purpose of the drum brake hydraulic system having some residual pressure is to keep some pressure level against the brake shoe return springs so they are faster to respond when the brakes are applied.

FWIW, rebuilding a Master Cylinder is fairly easy to do if you have the correct kind of wheel cylinder hone, and plenty of brake fluid to help keep the cylinders and stones wet/lubricated. When refilling the Master Cylinder use DOT3 (not DOT 2) brake fluid as it has a higher boiling point than DOT2. Do not mix the DOT 3 brake fluid with newer brake fluids, ads they may not be compatible. I have heard some are compatible, but I prefer sticking with the original specs as the rubber parts are definitely made for DOT 2w or 3. Also, do not introduce any kind of petroleum oil fluid into the Master Cylinder or other brake parts, as it will damage the rubber parts, badly. And moisture is something that loves to intrude DOT 2/3 fluid. Keep the cover of the Master Cylinder closed any time you do not need to leave the cover off (filling the reservoir).

Finally, not that it sounds like to need to replace or rebuild the front brake calipers, but if you ever do remove the calipers it is physically possible to put them on the wrong side of the car. Doing so will make it impossible to properly bleed the calipers, as you will see in the attached photo. Forewarned is forearmed.

 

[73inNH]

Great info.

 

[Bentworker]

Those wacky ford rear ABS units are problematic. There is a little accumulator in them and some other mystery bits. Dirty brake fluid can destroy them. If the master cylinder does not fix it swap out the ABS unit. Rock Auto has them.

 

[73inNH]

I should've updated this, sorry.

Turns out it was a driver's side front brake line leaking. The MC and ABS unit were ok.

Thanks for all the advice!