[hweaver]

My 90 F350 CC 4X4 has not been operated very much since last summer, but this weekend I took her for a drive. Everything was working well except the Banks waste gate was still hanging up and trying to over boost, but that is another problem. Suddenly when I applied the brakes the pedal went to the floor without any pressure or stopping. Lots of pumping got her stopped. I checked the brake fluid, and it was clean and full. After several attempts the brakes started working again, but I can press the brake pedal and the pedal moves slowely to the floor, if pressure is held. Some history follows:

- last year I replaced both the master cylinder and Power brake booster. I bled the system repeatedly and used lots of brake fluid, but never really got what I consider good brake pedal pressure.

- I can't lock up the tires, even in panic stops. I have never been able to do that in this truck.

- The reason I replaced the brake master cylinder and booster was because the brakes suddenly failed and then started working again later (same as this time). I was in some of our smaller mountains on a dirt road when that happened, and it really got my attention(scared the poop out of me and my wife).

What is the likely cause of this problem?

Thanks in advance.

hweaver

 

[Weezer]

hweaver,
Been there, done that, got the T-shirt. Two things worked for me (I too replaced the M/C, both front calipers, rear drums, shoes, cylinders): got much of the pedal back after properly adjusting the rear shoes up, got the rest of it after bypassing the RABS valve- I think there was internal (infernal) leakage. I just got the necessary fittings and went around it. Stupid design anyhow- who cares if your rears lock up /ubbthreads/images/graemlins/shrug.gif? I'm more concerned with the fronts locking. Seems to be a common malady with these things- good luck,

Keith

 

[cdnsarguy]

[ QUOTE ]
who cares if your rears lock up

[/ QUOTE ]

YOU WILL on a rainy interstate panic stop when your doing circles at 65mph. The brakes (RABs)are designed to keep the braking/tire traction as required for a SAFE straight stop with all loads.

 

[icanfixall]

If you bypass this and swap ends on the road causing an accident just pray the ins. co. dosen't look under the truck and see what you did. I know most adjusters wouldn't look that deep into things but why take that chance.

 

[Weezer]

Pete,
I understand what you're saying, it's just in my opinon that this version of ABS is an engineering abortion and mine never worked right from the get-go. I travel with 4000 pounds on my back over 95% of the time. I'd rather have the pedal than the mush I had. But you're right, I prolly ought to try to make it right, but it works oh so nice now!

Keith

 

[hweaver]

I guess I am confused. I didn't know that my 90 F350 had RABS. I didn't know that it had any type of anti locking brake system at all.

The brake problem happened setting still or going slow, so would the RABS (If I have it) be affected then?

You guys are all smart about these trucks here, so all help appreciated.

Thanks

hweaver

 

[cdnsarguy]

hweaver.......inbound email

If you had no brakes and no known fluid loss from the reservoir I would suspect the M/C. Quite often if there is no braking reaction on a medium pedal effort, but increased pedal effort does allow braking then the M/C cups are suspected as the problem. Even on brand new rebuilt M/C there are occasions of the cups allowing fluid past the cups. How to tell.... if the fluid level stays the same when the pedal is pushed and no braking effort is afforded the cups are not contacting the barrel correctly. If the brakes become useful with rapid pumping then the cups are flaring out and contacting the bore and thusly putting pressure to the brakes, there should be a fluid level drop in the M/C.

If there is a fluid loss from the M/C but non visible from any wheel/cylinder/line consider the booster as fluid can go into it from the back seal. NOTE some seeping is normal lubrication of the M/C piston/seal found at the rear of the M/C.

 

[gufman]

it may be the pump that creates the necessary vacuum for the braking system.

 

[cdnsarguy]

gufman that could be possible too, so visit my gallery for the vacuum test.

 

[Mr_Roboto]

A bad vacuum pump would cause a hard pedal, not a pedal going to the floor... The vacuum has nothing to do with operating the brakes, it just powers the booster, which increases your foot pressure to the master cylinder.

 

[tim94f250]

Did you bench bleed the master before installing? Sounds like you never fully removed all the air. Tim

 

[94_turbo]

as roboto said--about the vacuum--also--since do mech work--dont want to tell of the number of bad NEW--and rebuilt m/c's ive run onto--and they dont pay my labor to replace either!!!! /ubbthreads/images/graemlins/mad.gif me say m/c bad--and make sure you bench bleed to get all the air out--some of them can be a cockroach----

 

[Mr_Roboto]

94 Turbo makes a good point. When you buy a rebuilt cylinder, basically you are buying the crap that someone else traded in as a core, and was built as cheaply as possible.

Not saying there aren't good rebuilds and re-mans, but you can get junk too....

 

[exekiel69]

Pete do you have any info on how to bench bleed the master cylinder i might have to replace one soon.

thank you, Exekiel.

 

[tim94f250]

When you buy a master cylinder, it should come with two plastic fittings that screw in where the brake lines connect. It should also have two short pieces of clear hose. the plastic hose pushes onto the fittings. You need to fill the master with clean brake fluid, and loop the clear hoses into the brake fluid. Now you either need to hold the master, or carefully clamp it in a vise. Now you take a blunt punch, or something similar and slowly press the piston in and out until no more air bubbles appear from both hoses. When you get all the air out, top off the fluid and install the cover. If you do it properly, you won't even need to bleed your brakes. If for some reason it doesn't come with the fittings and hoses, you can make your own buy buying two of the shortest brake lines at the parts store and just bend them into a loop.

 

[icanfixall]

I just install mine on the truck, then hook up the plastic supplied hoses from the fitting back into the resovoir. Push down on the brake peddle till I don't see air bobbles anymore and then hook up the steel brake lines and I'm done.

 

[hweaver]

I did bench bleed the master cylinder. I replaced the M/C and the booster individually, not as a unit, but I think I had the rod adjusted properly. It probably is the M/C.

I definitely don't like RABS, so I will look into disconnecting that. I had RABS on a 90 4runner and on icy streets it wouldn't stop at all. I had to shift into 4 wheel drive any time I needed to stop quick, because that disabled the RABS. Eventually I unplugged the system. I had to look at the brake warning light, but I didn't look at the rear of the car in front of me so close any more.

Thanks for the help folks.

hweaver

 

[cdnsarguy]

[ QUOTE ]
but I think I had the rod adjusted properly.

[/ QUOTE ]

Herein may lie part of the problem, a bad adjustment would not allow the M/C piston to return to the correct location.

 

[hweaver]

I guess I will have to check over my work. Can I assume from your statement that a slightly loose rod is probably better than a slightly too tight rod?

hweaver

 

[hweaver]

This is an update on the brake problem that my truck had. Ultimately I replaced the master cylinder and unplugged the RABS. Now everything seems to work correctly.

I got the original M/C from NAPA almost 20 months ago. I found my receipt, and took the M/C back to NAPA. My warranty had expired long ago, but they replaced it anyway. KUDOS for NAPA.

hweaver