Have a 69 f100. Before i messed with the brakes, it braked ok, but i had two leaky wheel cylinders in the rear, so i put all new brakes and cylinders in the back. After that, it braked fine for a day, then i started losing pressure as i would press the brake, and then if i let up on the pedal i would lose all pressure. So i replaced the master cylinder. Now were still having the same problem. I can't seem to find any leaks on the cylinders, and the front cylinders were fine. So what are some things i should start looking at? Thanks.

 

If there are no visible fluid leaks, it sounds like to me the MC is internally bypassing --the compensating port inside is not sealing off and is allowing the fluid to dump right back into the MC reservoir instead of building pressure.

It doesn't matter if the MC was just replaced, they are frequently bad right out of the box --particularly if it's a rebuilt MC.

If you didn't buy a brand new MC to start with, I would advise to do so this time. Personally, I don't fool with rebuilt MCs.

Additionally, whatever parts store MC you buy, new or rebuilt --most particularly with a rebuilt, it's a good idea to remove the primary and secondary MC pistons to wash out the MC bore, before you install the MC.

Most if not all of these MCs are produced over seas and quality control isn't always tight. Often, there's machining residue left in the bore. This is abrasive and eats up the piston seals causing the MC to bypass.

If you look at the back of the MC (where the input rod goes into it) you'll see a large snap ring. Remove it and you can then slide the primary and secondary pistons out of the bore. Set the pistons aside in a clean area.

Wash the MC bore out with acetone and blow dry. Wipe the seals of the pistons with a lent-free rag. Coat the inside of the MC bore with brake fluid. Coat the piston seals with brake fluid and carefully reinsert the pistons into the MC bore, being careful not to nick or tear the seals and re-install the snap ring.

This step is simple and doesn't take long to do but, it could be the difference in the MC lasting years vs. minutes, days or weeks.

After this, bench bleed the MC, bolt it on and plumb it up and you're ready to finish bleeding out the rest of the system.

 

This is the kind of experience sharing that I appreciate so much from this site. :-)