Intermittently NOT STOPPING! PLEASE HELP - Diesel Forum

paripam1980 · 10-18-2009, 11:36 PM

I have an 87 F250 4x4 excab with 7.3 and banks sidewinder. About 2 months ago I was running around town unloaded during normal stop and go traffic. At one point I had to make a "sudden stop" (just short of locking 'em up). All was well until I came to the next signal light and the pedal went ALL THE WAY TO THE FLOOR. THERE WAS NOTHING. (except a load in my pants) lol. Pumping did nothing. Thank you to well adjusted and functioning parking brake. I pulled over and checked things out. No fluid loss, no leaks, no bulging hose, nothing. The truck sat for about 20 mins. I then stepped on the pedal again and it was back to normal. No pumping or anything. It was like nothing ever happened.
So, I figured it was the master cylinder, and replaced it with a NEW raybestos unit. Bench bled and did 2 man bleed all wheels--was working great. (or so I thought)
A few days after I went up the hill to drag home the 27' house trailer from the mountians. IT HAPPENED AGAIN-GOING DOWN THE 15 MILE LONG 6% GRADE!! Thank the Good Lord above for a good parking Brake, good working trailer brakes, and low gearing. Again, after 20 mins or so it was like nothing ever hapened. I am at a loss??? I have lost faith in this truck and need to get this issue resolved. Please any advice......

Thanks John

cdnsarguy · 10-19-2009, 05:51 AM

Sometimes the worn seals in an old master cylinder will not flare out under pedal pressure.... That could cause your problem. However after the replacement one would think it would be fixed. The check for the seals/M/C is to press the pedal slowly if it goes to the floor, press the pedal fast several times and it may build pressure if it does replace it. If the vacuum pump was bad you would have had a hard pedal with decreased brake effect.

M.L.S.C. · 10-19-2009, 09:01 AM

I'm thinking it could be the RABS HCU failing? '87 was the first year for those, and I don't know if there were any revisions made to them before the full 4-wheel ABS was introduced in the late '90s, but there are some valves in the HCU and there are cases where one of them leaks past its seals and when you step on the pedal the pressure you build up in the brake line gets relieved in the accumulator chamber. What happens is the fluid gets pushed past the valve and in the chamber and against the spring-preloaded piston when you step on the pedal, and when you let off the pedal the piston pushes the fluid right back in the lines and up to master cylinder - no fluid lost, however no pressure buildup either. And here's the bad part - I have no idea as to how to test the RABS HCU for proper operation, or even if the test-procedure for its electronic controls can help you identify a leaky valve... Unless! The RABS relies on a VSS in the rear axle to tell it when the rear wheels have locked up, zero wheel speed reading commands the line pressure purge valve to open and keep cycling open-close open-close for as long as the wheels stay locked. Whether a faulty VSS could trick the RABS into going wide open with said valve and thus dumping all your rear line pressure for long periods of time, that I don't know either, but I'd imagine some member here may have that information. If you had the E4OD trans a faulty VSS may show as an abnormal shift schedule, but in '87 the only automatic was the C6 and it don't care for the VSS so it really matters none whether you have a slushbox or a bolt-action trans, and your speedometer is cable-driven too, so really you have nothing that can indirectly confirm a failing VSS.

paripam1980 · 10-19-2009, 11:39 AM

I thank you guys for the insight.--If the RABS was dumping the rear fluid pressure into the accumulator, wouldn't the proportioning valve see that as an inbalance in the system and block the fluid to the rear, and activate red "Brake" idiot light? Thus leaving me with front brakes?
Anyway, Im going to check the RABS. The ABS idiot light goes through bulb check, but other wise is not on indicating a fault. VSS seems ok- cruise control works (except around 65-70mph it likes to surge, but I figured that might be a problem with the servo)
As with most automotive issues, the intermittent-ness of this problem makes this so frusterating.

M.L.S.C. · 10-19-2009, 12:00 PM

These trucks don't have a proportioning valve, that's the beautiful thing about them - the front and rear circuits are entirely isolated from each other, and if you have a large two-chamber master cylinder reservoir even if you lose all the fluid in one circuit you still have the other one to at least slow you down.

But you are correct, if the truck had a prop valve the pressure drop due to the accumulator chamber filling up would cause the it to shut the fluid flow and trip the light... which is exactly why there's no prop valve, cause that would be happening every time the accumulator chamber starts filling, including regular RABS operation on low-traction road surface.

These trucks light up the red brake light under three conditions IIRC - when you put the parking brake on, when your vacuum pump takes a vacation down south and you lose brake assist as a result, and if you have a plastic-tank master cylinder with the float when the fluid level in it goes too low.

IIRC Pete here (the member who responded first in this thread) has the diagnostics procedure for the cruise control system, so he can probably confirm this, but I don't think the cruise control uses the VSS in the axle, only the RABS needs it.

LMJD · 10-19-2009, 12:26 PM

Quote:

but I don't think the cruise control uses the VSS in the axle, only the RABS needs it.

Right. Years ago Raybestos (or one of the other big brake component companies) who made the original RABS valves for Ford came out with a service bulletin saying they were subject to chronic failure. I'm on my 3rd one in 18 years so I'm inclined to agree. However the bulletin described the failure symptom as a very slow pedal leakdown under constant pressure like waiting at a red signal lite. Same symptom as a bad master cylinder. That was my exact symptom each time. I took one apart once years ago and they're kind of an engineering trainwreck IMO. By design it doesn't seem like it could cause a sudden loss of pedal, but maybe so. Even if it got a error message saying rear brakes were locked up I'd think you'd get a pulsating pedal like when they really are. What say you, M.L.S.C?

Paripam1980, did you look EVERYWHERE under the truck for a fluid leak, each wheel backing plate, etc? If it was the master cyl you may see fluid running down the firewall after it leaks into the booster or on the inner firewall when you pull back the carpet/padding inside near the brake pedal pushrod area.

paripam1980 · 10-19-2009, 12:28 PM

right on--I will check into that. Thank you for the clarification. If the master cylinder is "split" wouldn't I still have front brakes when this happens? Cause when it craps out-the pedal goes to the floor. I personally feel that the new master cyl may be a defictive unit, but I need to explore ALL AVENUES--so all of this info is SO VERY HELPFUL. Thank you.

LMJD · 10-19-2009, 12:38 PM

Quote:

--so all of this info is SO VERY HELPFUL.

Not really, cause we're ALL confused too!

Just kidding, you got a real weird one goi'n on.

Quote:

If the master cylinder is "split" wouldn't I still have front brakes when this happens?

Duh, yes you're right, which make the situation even more odd.

paripam1980 · 10-19-2009, 02:05 PM

Yes--This is quite confusing. I keep thinking that I am missing something--This week I will go through the whole brake system with a fine tooth comb. I will keep you posted. Please keep the suggestions coming if anyone thinks of anything.

cdnsarguy · 10-19-2009, 02:22 PM

One question has the braking effect come into play at the point where the pedal is almost to the floor..... and just about tossed you through the windscreen.

Did you by chance remove the adjustable pin in the booster at any time..... during the master cyl replacement....................do not ever do that.

paripam1980 · 10-19-2009, 02:31 PM

No- did not mess with the pushrod in the booster. Just bench bled the new unit and installed, followed by 2 man bleed at all 4 corners in order RR, LR, RF,LF.

M.L.S.C. · 10-19-2009, 03:58 PM

Quote:

Originally Posted by LMJD

Right. Years ago Raybestos (or one of the other big brake component companies) who made the original RABS valves for Ford came out with a service bulletin saying they were subject to chronic failure. I'm on my 3rd one in 18 years so I'm inclined to agree. However the bulletin described the failure symptom as a very slow pedal leakdown under constant pressure like waiting at a red signal lite. Same symptom as a bad master cylinder. That was my exact symptom each time. I took one apart once years ago and they're kind of an engineering trainwreck IMO. By design it doesn't seem like it could cause a sudden loss of pedal, but maybe so. Even if it got a error message saying rear brakes were locked up I'd think you'd get a pulsating pedal like when they really are. What say you, M.L.S.C?

Yeps that is indeed the common issue with these units, it's the valve seals causing an internal leak into the accumulator chamber. And I agree that if the VSS is causing it to think the wheels are locked up it should cause a pulsing pedal, that's actually where I get totally confused - it almost seems like his line pressure purge valve goes wide open and stays open for quite some time, instead of cycling on/off like it should during normal ABS operation, but what could cause it to do that? Maybe a short in the wiring somewhere?

Quote:

Originally Posted by paripam1980

right on--I will check into that. Thank you for the clarification. If the master cylinder is "split" wouldn't I still have front brakes when this happens? Cause when it craps out-the pedal goes to the floor. I personally feel that the new master cyl may be a defictive unit, but I need to explore ALL AVENUES--so all of this info is SO VERY HELPFUL. Thank you.

You should have at least front brakes, in theory, however I've noticed that the pedal will go way too far down when one of the brake circuits fails, thus messing up with all pressure rises you may be trying to build up in the system. That said, I've always been able to stop the vehicle, which is where your situation gets really weird, if you did indeed lost all braking abilities... Are you sure all your brakes were gone? Truck not even trying slow down, just keeps coasting? I'm really asking about your first incident, the second time you had the trailer so all bets are off there.

Quote:

Originally Posted by LMJD

Not really, cause we're ALL confused too!

Just kidding, you got a real weird one goi'n on.

yah, no joke, the more I think about it the more confused I get

paripam1980 · 10-19-2009, 05:55 PM

there may have been something (some brake response) but not enough to remotely even come close to slowing down the truck. (1st incident, unloaded)
Does anyone have a good schematic of the RABS system both electrical and possibly hydraulic?
What if I dissconnect the electrical from the RABS valve and make test jumpers to manually activate the solenoids-in an effort to re-create the problem and see if I get the same response? (pedal to the floor) That way I can try it in a controlled enviroment (my shop) instead of out on the road.
Does this make any sense?

M.L.S.C. · 10-20-2009, 10:24 AM

Yes, your idea makes perfect sense, but I don't have the electrical diagrams that you need...

paripam1980 · 10-20-2009, 11:23 AM

Well, I have done a little investigation last night. First, I made a jumper and manually activated the dump valve in the RABS. The pedal dropped a bit but not like before.
Next, I opened up a rear bleeder screw to simulate a a huge leak. Again, low pedal but still had some hard pedal--just further down.
Last, I opened up a caliper bleeder screw to create a leak in the front.
JACKPOT-The problem was duplicated.
I only had time to remove the left front last night. I found that the brakes got really hot at some time. This confuses me because I am one to always be very careful with my brakes--always use gearing when possible.

Maybe calipers are holding? Is that check valve looking thing in the master cyl between the master cyl and front brake line holding pressure? What is that for? Keeping the line "charged"?
I have a meeting tonight, so I don't know if I will have time to check the Right Front today.

MORE TO FOLLOW