Hey guys I have had a sinking pedal all summer. No the pedal does not pump up to gain stiffness either. If I hit the brakes really hard and fast it gets hard for a split second then sinks back to almost to the floor. Now that we just had our first snow I quickly realized only my front breaks work. I could not stop to save my life. None of my lines leak but the right rear axle seal leaks. Ive replaced the wheel cylinder back there because I thought that's were it was leaking. Does this sound like a master to you guy's or what? Any help would be appreciated because I would like to get this fixed asap

 

Not sure if I can help, but will try nonetheless.

Since you recently replaced a wheel cylinder, I'm assuming you bleed the system of air (including RABS) and kept the master topped off?

Bar no fluid leaks in any brake lines, have you also checked vacuum at the brake booster? I also assume the trucks not diesel right? (if it is you may want to check the vacuum pump too.)

Usually if you have no brakes, it's hydraulic in nature...meaning you could have a bad master. But, I would perform some basic checks on it before replacing anything.

Hope you get the issue tracked down soon...scary driving around without brakes.

 

Trip2way, what does RABS stand for again? I always forget .

 

it sounds like your RABS (rear antilock brake system) is not working.
there is a valve on the frame about under the driver seat.

it has been bypassed on my 88 since the trucks was a few months old because it never worked properly from day one, and ford could not figure out how to fix it.
the brakes were hit and miss, they had a mind of their own. one second you would kiss the windshield when looking at the brake pedal, 10 seconds later you would be running a red light because the truck had no brakes.
so i just eliminated it.
494,000 mile on it now with no more brake problems.

 

Okay. Yes, RABS stands for Rear Antilock Brake System valve...or some call it the HCU (hydraulic Control Unit).

Now, if your a DIY kinda guy (like me) please do some research or at least partner with a skilled mechanic (I'm not), and some searching here at FTE before you modify anything.

You'll see a lot of advice, some good, some bad. But, the more info/knowledge you gain will help you and cover your ***.

I strongly suggest NOT to bypass any safety devices on your F250.

With that said, do some basic troubleshooting first. Narrow down the issue with no brakes in the rear.

Let us know what you find. Good luck.

 

start with the basics.when you feel like you have no rear brakes,then pull the drum and inspect the condition of the shoes.they don't last forever.your going to have a sinking brake pedal if you have a leak or vac issue.

 

I have a 302 so no vacuum pump. And I was going to pull the master off today to see of I had any fluid back there. What are some more things I do to narrow the issue down

 

my reason for bypassing it is i never had a vehicle with ABS, so not having it is not a big deal for me. and the truck was more dangerous with the abs hooked up than it was without it.

 

My trailer pulling rig, 1990 f-250 4x4 4.9 5spd has same issue i.e. variable braking performance ranging from poor to bad.

After extensive research I learned when Ford added RABS to F-series trucks they did not redesign the entire brake system. Instead, as a retro fit:
*sensor was added to rear axle
*control module was added (receives input from rear axle sensor & sends signal to ABS hyd unit)
*ABS hydraulic module was added (control brake fluid to rear brakes)
Everything else in the system is essentially decades old technology.

You will find many, many "nay sayer's" concerning deletion of the RABS on your truck. There are also many, many who have by-passed the RABS with no ill affects.

My DD is 1995 F-150 4x2 4.9 5spd with exact same RABS system. Last spring it began exhibiting alarming brake performance. Since it is my DD it was essential to find/repair the issue.
Narrowed it down to the RABS solenoid module on the frame. RockAuto.com offers after market "ABS hydraulic module" in $150 range +core fee.

I made decision to bypass RABS on my 95-F150 after a lot of reading on FTE as well as examining my brake system and realizing that RABS was a factory retro-fit rather than integral to the system, . My 90 F-250 will get same treatment next spring.

On FTE I read several post where the prevailing opinion was "don't bypass it, fix it because it may be dangerous. . ." with no factual experience to back up "dangerous". Also read many post of actual experience stating that no problems were encountered from deletion of RABS.

Shared all that to say this: Im a firm believer in "as designed is best" logic however, in the real world practicality must be considered.

Use your own best judgement. Do not forget, FTE is not an "expert" conglomerate-just a bunch of Ford enthusiast (with widely varying opinions) who love their trucks and wish to share their experiences and knowledge. Always remember, "expert" advice is never free!

Is it worth the expense to restore factory function to a 24 year old system? Up to you. My 2cents? RABS deletion was right decision for me!

 

Drove truck (95f150) for approx year prior to replacing front pads which is when I discovered some sinister problems. Used following procedure revealing bad ABS hyd unit.
I knew fluid was not going to rear brakes when open wheel cyl bleeders had no fluid flow.
Next:
*Pulled rear drums and like new shoes were revealed with no fluid leakage present.
*Examined lines up to RABS module on frame, found short length or rubber hose rigged in for rusted line repair-replaced line.
*Removed line from ABS hyd unit and no fluid to rear line,
*removed line from master to ABS hyd unit, no fluid to ABS hyd unit-suspect master cyl.
*Began removing mastyer cyl by removing check valve 1st and discovered PO had blocked it off internally with a walmart bag, yikes! (master cyl ok)
*Replaced check valve and lots'o fliud to ABS hyd unit, still no fluid to rear line
BINGO-bad ABS hyd unit.

 

that is another thing. everyone keeps on telling me i have an abs tone ring in the rear. i do not. my 88 has a very simple system. the only truck that had the tone ring in the rear was the 02, and that was for the speedometer.
the two 88's, 89, 90, and 91 all had the same one wire ABS system.
maybe because they were all F-350 4X4's with diesel and C-6 trans.

 

I recently had the same issue with sinking pedal. Would stop, but then slowly sink to almost floor. It was the master cylinder. Replaced and now all is good!

 

Ok, I have one more thing to add about the brakes, when I step on the brakes really hard the idle goes up does this mean I have a bad brake booster? Im not really good with brakes at all so if anyone can chime in on how to track this down id appreciate it. The pedal goes all the way to right above the floor, but is firm down there it doesn't slowly sink the rest of the way down.

 

Okay. Thanks for following up. Since you say "I'm not good with brakes", I would suggest you take the truck to someone who is. It doesn't sound like you've been able to perform basic troubleshooting, but maybe you have and just not articulated that here.
I'll make some assumptions to help, but you'll need to have checked the specific parts discussed.

For a low brake pedal as you described you could have the following problems that will need to be eliminated or ruled out.
1) Low or contaminated brake fluid. Once full and bleed of air, this shouldn't be a problem, unless you have a leak somewhere. When you pulled the master cylinder from the booster , was there a leak? Any signs of fluid contamination?

2) Worn brake pads and/or shoes. Have you visually inspected these?

3) Bad power booster or check valve. Need to check engine vacuum at the hose running from intake to booster check valve.
If low ( less than 16 inches) you may have more than brake problems. If vacuum checks good, then test the vacuum check value at the booster. It should only let air in from the rear.

Let me know what you find.

 

I'm good with anything besides brakes. I've done 2 engine swaps and both were when I was 15, I'm 16 now and just really haven't learned a lot about brakes yet. I know the basics, like how to bleed the brakes(which I've already tried) and I did look at the rear shoes which seemed fine although I've never replaced them so maybe I don't know what fine is. Could a leaking axle seal in the passenger side cause this issue? I've already changed the wheel cylinder because I thought that's where the black gunk was coming from. Ive never thought the fronts could be the issue because there the only brakes that work. I plan on pulling the the master off of the booster this weekend to Check for fluid.