I have read all of the posts on brakes and have done many brake jobs, but I am having problems. I have put on new front brake hoses, new front caliphers , new pads, rear shoes and wheel cylinders, turned rotors and drums and new 1 5/16 Bendix master cylinder. I have gravity bleed, assistant bleed and vaccumed bleed the brakes and ran almost a gallon of Dot 3 through the system. I have installed a new vaccum pump and I still have a petal that goes to the floor. I had installed two 1 1/8 master cylinders before the larger Bendix unit and the petal was the same. The only part I have not replaced is the RABS unit. Can the RABS unit cause the system to act like there is air in the system. The are no ABS lights coming on and I have unplugged the rear sensor and wiring harness on the back of the RABS unit with no change in the petal , but the ABS lights do come on. The truck only has 65,000 on it. Thanks for any help.
88 F-350 Brakes
- Thread starter D W Coyle
- Start date
jauguston
Retired
I don't have any magic answer but at this point I would rig yourself the fittings necessary to cap your brake lines. Start close to the master cylinder and cap both lines. If you get a hard pedal move to the next connection and cap there. It will be a PITA but you all ready have a PITA. That should narrow down where the problem is.
Jim
Jim
krawlr
Yup, that's me
Are the rear brakes adjusted? Only asking since you didn't mention it.
Was this a problem before the brake work?
ABS systems are always designed to fail passively as in back to non ABS brakes.
Was this a problem before the brake work?
ABS systems are always designed to fail passively as in back to non ABS brakes.
Update on brakes
I just plugged both master cylinder ports,I have a good hard petal. I put the front line back on and petal goes down , but still a decent petal. I then plugged it and put rear line back on and petal goes to floor. So it is definitely mostly on the rear. Someone had posted that there was a bridge by Ford that could be used to bypass the RABS unit, but he was not a home and did not have the number. Does anyone have that part number ? I will pull the rear drums again and check all of the new parts, but would like to isolate the RABS unit to make sure that is not it since I have not heard back that it could be the RABS.
I just plugged both master cylinder ports,I have a good hard petal. I put the front line back on and petal goes down , but still a decent petal. I then plugged it and put rear line back on and petal goes to floor. So it is definitely mostly on the rear. Someone had posted that there was a bridge by Ford that could be used to bypass the RABS unit, but he was not a home and did not have the number. Does anyone have that part number ? I will pull the rear drums again and check all of the new parts, but would like to isolate the RABS unit to make sure that is not it since I have not heard back that it could be the RABS.
jauguston
Retired
I don't know what a RABS is but if you are talking about the valve that is mounted on the rear axle and you have drum brakes on the rear I know what that does. That valves purpose is to prevent rear wheel lock-up and skid when there is little or no load on the back and you are making a sudden stop. The link that connects to the frame senses how much the springs are depressed. If they are not depressed very much the valve has a pressure regulator in it that prevents normal brake fluid pressure from getting to the rear brakes. It is not a proportional valve, it is either a straight through passage or if light loaded the pressure regulator is in the circuit. On my MH the linkage was in the way when I put dual 3" exhaust over the axle and exit behind the wheels. I simply removed the linkage and rotated the valve to the position it would be in with a big load as I am always loaded with the MH. The only thing that I can see that that valve could do that would be bad would be leak if the linkage is working properly.
Jim
Jim
akoldnav
Full Access Member
I had similar problems with my 90 F350 CC. I unplugged the RABS unit and that seemed to help some. I still have a softer pedal than I like.
akoldnav
akoldnav
f-two-fiddy
Registered User
Does the 88 have a brake proportioning valve on the frame or is it in the MC?
79 300sd
Registered User
krawlr
Yup, that's me
"ABS systems are always designed to fail passively as in back to non ABS brakes"
I got that from my Bosch training.
More proof those trips to Chicago were a waste of my time. 
I got that from my Bosch training.
More proof those trips to Chicago were a waste of my time. 
Double-S-Diesel
parts seller
possibility of a bad master cylinder, I know you have put 3 on it, but I just went through the same problem with a caravan, lots of new parts, including abs pump, and 3 master cylinders, after 3 days, went to the junk yard, took off one that was working, put it on the van , problem solved.
Just because a part is new doesnt mean it is good, kida sucks for a mechanic.
Nick
Just because a part is new doesnt mean it is good, kida sucks for a mechanic.
Nick
Mont91
Full Access Member
The '88 and up have an electricaly controlled Rear Antilock Brake System(RABS) control valve mounted in the frame rail under the driver.
Prior to htat it was a mechanical valve mounted over the rear axle with linkage to the rear axle.
Did this problem show up after or before you started on the brakes?
Did you adjust the rear brakes per specs? This is very important on these.
Prior to htat it was a mechanical valve mounted over the rear axle with linkage to the rear axle.
Did this problem show up after or before you started on the brakes?
Did you adjust the rear brakes per specs? This is very important on these.
oldmisterbill
Grumpy Old Man
I had a problem with my brakes in my 91 F350.It started with a spongy pedal-then a collapsing pedal.Next-replace master cylinder -a little better-I check brake adjustment several times-good.Got some brakes went on a trip lost brakes-bled them-got brakes-lost brakes took to Autozone -gave me a new master cylinder-got some brakes.FRUSTRATED!!!!lost them again -I went thru the replacement-bleed syndrome.Finally in St louis on my way home one night I stopped at a shop they replaced the mastercyl again.I had brakes-oops they went 200 miles later.I limped home-in frustration I bypassed the Rabs valve.I have great brakes -too great.The truck will lock up empty if I don't pay attention.But with my gooseneck it is wonderful-I have driven thru several blizzards and feelt safe.Now I drive ocasionally with out the trailer-I want a new RABS valve!! I think many times I have seen posts where modifications were suggested ,when the rela pproblem is the Rabs valve.My 2 bits
snatchal
Registered User
I am having the same problem with a sinking brake pedal. Could this problem be solved by replacing the ABS valve with an adjustable proportioning valve such as this one from Summit?http://store.summitracing.com/partdetail.asp?autofilter=1&part=WIL%2D260%2D8419&N=700+4294922533+400196+4294918719+115&autoview=sku
something i didnt see mentioned before that i have seen on other vehicles is how good are your rubber hoses. i have seen vehicles that the rubber lines swelled when the brakes were hit that caused a condition that you mentioned. i have had freinds note when they upgraded to braided stainless lines on there jeeps the pedal improved. another thing make sure two that nothing could cause the caliper to spring back out when the brakes are released. i have ran into this before with missplaced spring clips causing extra travel on the calipers which equals extra brake pedal travel.
Guntherx
Full Access Member
Hey everyone. It's been awhile since I have been around. I am researching a brake problem and didn't see anything quite like I have so far. The brake pedal went nearly to the floor, but I still have some brake to stop if I take it very carefully. I looked at the MC and it was empty. OK must have a broken line. Filled the MC and started pumping which usually has fluid squirting from somewhere and shows the leak. Nothing. I saw a TSB 24 from 1989 diagnosing the EH valve which states fluid can go into the accumulator. My question is can it leak from the accumulator? I was also wondering if it is leaking out the back of the MC can it be sucked up by the Vac system so there is no evidence? Just trying to minimize troubleshooting as time is limited due to serious family health issue.
Any thoughts are all appreciated.
Any thoughts are all appreciated.
