Brake fade

[Squirlye]

It's been awhile since I've posted, but I've kept an eye on the forum the best I could.  I still have a ton of work to go on this truck. Slow going, but i have been using it as my daily driver for a while now. So now for my problem.
The brake pedal will almost go to the floor when stopping. The weird thing is, when down shifting the pedal becomes firmer with the higher rpm's. Then fade when I'm at a complete stop. At first I just thought I just needed the brakes to be bled. So I had the wife help out. The fluid was definitely bad, it was very dirty almost black. She did the best she could, the brake pedal was still a little softer than I'd like. A couple days later the pedal was really soft again. When you start it up, if you have pressure on the pedal it will go almost to the floor. I'm not leaking brake fluid anywhere. Any ideas or suggestions?

[viven44]

Sounds like air in the line (at least part of the problem) .... and I understand you have tried bleeding it.

Do all 4 wheels gravity bleed well ?

[ArdWrknTrk]

Sounds like fluid is bypassing the master cylinder piston and returning to the reservoir.

O-rings eventually give up, especially with sludgy fluid.
You have a firm pedal, when you have little vacuum.
Do you hear a hissing sound from up under the dash when you apply the brakes?

[mat in tn]

that really sounds like a failing master cylinder

[Rockhauler]

I'm just in the process of going through my truck from rebuilding it many years ago. Part of that is checking my brake components as the brake fluid was dark and nasty looking. Took wheel cylinders apart found both had one of the pistons seized and lots of gunk. Then figure lets see what the master cylinder looks like it had lots of gunk around the piston/spring and seals. The bore didn't look good either so I just replaced the calipers, master cylinder and the wheel cylinders as I like good brakes and yes all new brake fluid.

I would expect your brake system might be the same if it hasn't been touched in many years. Since mine were still sitting on the bench here's what they looked like,

 

[mat in tn]

I'm convinced that nobody flushes their brake hydraulics as often as necessary. not even myself. but yes, it should be done annually. otherwise, the moisture absorbed by the fluid rusts out the system from the inside out.

[Rockhauler]

Yes brake fluid absorbs moisture from the air so flushing would be a wise idea. Likewise I believe few people flush the brake system unless the brakes give trouble, me included.

Personally every master or wheel cylinder I've ever taken apart the majority of the damage to the bore has been in the bottom. I doubt flushing would remove much of this gunk accumulation since bleeding draws from the top of the bore. It would be interesting to see how much gunk remains in the bores after flushing if one was to pull the pistons.

I just tend to replace the brake fluid, hoses, calipers, wheel cylinders and master cylinder about every 10 years.

[mat in tn]

and how many people even keep a car that long anymore? I know I'm an oddball, i bought my first truck in 1991 and I'm still driving it.  gotta go, I got brakes to bleed!

[85lebaront2]

Mat, I bought Darth April 1994 and have owned him ever since. I kept my Shelby for 11 years and only sold it because I had three kids and needed the money.

[mat in tn]

kids!!!!

[Rockhauler]

Ha odd ball guess I'm in the club not sure which year but bought my first truck in the early 90's and my second one in early 2000. Still have and drive both and no plans to replace either, just every 10 years or so freshen them up.

[viven44]

I don't know how y'all felt buying those trucks new back then... but today's trucks are not for the faint of heart with the amount of custom technology in them (OEM chips/modules only -- no aftermarket replacement) and the cost of simple repairs in the future can be high due to monopoly of parts. From that perspective, I feel like I missed the boat on owning a new truck and using it for a long time.....

[Squirlye]

Thanks guys for all of your advice. I plan on redoing whole brake system.
I'll start with the master cylinder for now, that way I'll be safe out on the road. My goal for the truck is to have a new old truck. Completely as reliable as possible. I'm not a fan of these new computer controlled vehicles.

[Gary Lewis]

If you are replacing the master cylinder I'd recommend going with the later style with the aluminum body and plastic tank.  We can get you the part number for one that doesn't require any plumbing changes save for bending the lines slightly to put the front in the back and back in the front.

One major advantage is the ability to easily bleed the whole system.  Get a 2nd cap, put an air fitting in it, and put ~3 psi of air on the master.  Open the bleed valves until you get no bubbles, and you are done.  (Obviously don't let the master go dry.)  And you can create that much air pressure easily with a garden sprayer if you don't have an air compressor.

[ArdWrknTrk]

U-haul surplus still had one listed last week, but I think it's the big bore 250/350 model.

They ship fast, and if course there's no core charge.  👍

Edit to add:  https://www.ebay.com/itm/173280150780?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=S3BjJ0E7SWC&sssrc=4429486&ssuid=V52-0s-ZTpm&var=&widget_ver=artemis&media=COPY

$52.18 + $4.95 shipping

[ratdude747]

The newly-made Raybestos one I used when I upgraded wasn't all that expensive if memory serves. Also no core charge. Despite what Raybestos says, mine did have the hard-to-find valve thingy on it (along with the level switch, which I added to my brake warning circuit because I'm an overachiever!). Pretty sure I bought mine from RockAuto?

[viven44]

Gary, other than the bleeding advantages, do we know if the newer master cylinders increase pressure in the lines ?

[Squirlye]

Yes I would love that information. I was going to buy everything at once, then  take a weekend and replace everything. But I think I'll replace the master cylinder first.

[viven44]

It possible the mounting stud spacing may also be different between 1/2 tons (F150, Broncos), and the HDs (F250 HD, F350, +)...

[ArdWrknTrk]

Yes, that and the slightly larger bore makes the pedal stiffer.