TLDR; New Brake Master Cylinder fixed the problem.

Ever since I bought this truck, Ive experience a slowly sinking brake pedal. It was only detectable with more than normal brake pressure applied and it was very slow, maybe .5" per minute. It was completely drivable and safe, but if I was sitting at a stoplight, i could feel the pedel sink further and further down. The first thing I did was to buy a Motorcraft rebuild Master cylinder off Rockauto. It was pricy at about $140. It didnt fix the problem. Then I dug into the problem a little further and I found many articles complaining that the RABS module is known the leak. So I did some diagnosing, by-passed it with a few brass unions and elbows, and that didnt fix the problem either. I bought a rebuild unit and replaced the RABS out of good measure. Brakes still sank. Rear brakes needed replacing so everything in the back got replaced. Rear Brake hoses were replaced. Brake pedal still sank. Somewhere through the years, one of the front brake lines sprung a leak, so both front brake hoses were replaced. Pedal still sank. I heard of a trick of installing brass flare fitting plugs in the master cylinder ports to check for bleed-down/internal leakage inside the master cylinder. The pedal was rock hard and didnt bleed down. I moved my search else whare. More research pulled up ford service Bulletin CQB 02-18 mentioning the brake booster may be the problem. It sounded like BS, but I ordered a used Zero-Loss brake booster on ebay which came complete with a no-name master cylinder. I removed the no-name master cylinder and bolted the Zero-loss brake booster to my truck with the original master cylinder. Brake pedal still sank. Running out of ideas, I pulled apart and rebuilt the front brakes looking for something obvious. Nothing was found so it was reassembled. Brakes still sank to the floor. At this point, the sinking brakes became alittle worse. Id need to pump the brakes at a stoplight because they were sinking out of my comfortable leg-extension range. Coming to a stop with a trailer down hill required a reset-pump to be comfortable. Out of desperation, I swapped out my ford master cylinder to my free no-name master cylinder that came with the brake booster. Suddenly, my sinking brake problem was fixed. Magic.

So why did the brass flare fitting plug trick show my master cylinder was good? My theory has to do with the "stroke" of the master cylinder. During the plug test, the pedal is rock hard and has almost zero movement. So the piston of the master cylinder doesnt travel down the bore very far... maybe only a 1/16". In normal usage, the calipers and wheel cylinders transfer a volume of brake fluid before they start applying pressure to the shoes and pads. At the point it start producing pressure, the piston is 1/2" into the bore of the master cylinder, which is where most of the wear and scoring happens. This scoring is probably leaking a small mount of fluid back into the unpressurized side of the cylinder, causing the pedal to sink down.

Items replaced while experiencing this problem.
Rear brake cylinder (motorcraft)
Rear brake shoes (motorcraft)
Rear brake hardware kit.
Rear brake drums (motorcraft)
Rear brake hose.
Rear brake ABS module.

Front brake pads x2 (Motorcraft and EBC Green stuff)
Rebuild front calipers (Motorcraft kit)
New front brake hoses (Raybestos)
Rebuild Master cylinder (Motorcraft)
Used Ebay "Zero Loss" Brake booster F5TZ-2005-CA (Ford)
Gallons of brake fluid.
Eventually, a free no-name master cylinder.




PItting corrosion and scoring in ford master cylinder.

 

Great write-up. Thanks for sharing the resolution of your issue.

 

with brakes, i never buy rebuilt always new.

 

YEP! That pitting is caused by water, brake fluid is hydroscopic..

 

I doubt it was caused by water. This has been a long term problem and the brake system has been opened, flushed, and bleed dozens of times through the years. I suspect it was a poor rebuild, although I have high regards for Motorcraft parts, this one might have slipped through the qa process or a counterfeit part from rock auto.

 

Bosch used to be the gold standard. Not much anymore.

I believe when the market is to demand low prices, companies have to comply or go out of business. The gold standard companies are not immune to this trend.

Hydraulic systems are pretty easy to figure out. Since fluid does not compress, the only way a pedal sinks is because there is fluid loss. Either internal or external. If the pedal drops and there is no external leaks, it's the master cylinder. Although I do my best not to buy things on the internet because of the issues you had, sometimes it must be done. Buying locally would allow a simple swap of the new cylinder. At least it would for me.

On the bright side, brakes are the most important part of the vehicle and when the vehicle gets old, it's best to swap the parts before they fail. It's not a waste of money to swap everything when they get old. Sadly, some parts are just NFG out of the box. New does not mean good. It means Never Ever Worked.

 

avoid the cheap rear brake hardware kits. mine was half ton, dont rememeber company, but the one side cable that actuates the lever which adjusts the shoes in reverse was the wrong legnth and the little lever which flips the adjuster was just wrong. if you used it, you would have brake lockup more than just in reverse and more than 1 click at a time. insane.