ok i have a 89 f250 351 zf 5spd.. 8600 gvwr.. been chasing out a brake gremlin now and im rackin my brains.. issue with it is that it has brakes... but just very little.. it doesnt catch tightly until about half an inch or so off the floor...

what has been replaced...

new brakes front and rear ( needed anyways )
new wheel cylinders (1 was leaking)
new calipers (bleeders were snapped)
thoroughly bled 4 times over..
thought it might be a faulty rabs module, instead of paying 160 if i didnt even know if it was the issue i instead bypassed it with a short jumper brake line to test results... same issue as started with.. so not a bad rabs module

i got to looking real closely at the brake booster... and im gonna feel dumb if its what it is, but question being with it being over 8500 gvwr i know the booster is supposed to be a substantially large unit... but looking at the one under my hood its a small one that i have in fact matched to a under 8500 gvwr f250...

now by looking at the master vs the booster.. it seems that someone replaced only booster and left master in place... so i am going with the 'assumtion' that the master is a over 8500 unit...

big question.. could a undersized booster cause all the brake issues im noticing... maybe the pushrods are too short to get the full pressure stroke earlier??

sorry for the 3 mile post but this thing is driving me insane lol

 

Safe to assume you have the rear shoes adjusted up to the drums? Sometimes over looked, will cause excessive pedal travel if fully retracted one or both sides.

And if its a re-man master cylinder take it back exchange it for a "new" one, even if followed the bench bleed process to a T will be lucky if it achieves a "solid" feel. If so your likely the only one ever got one that did!

You can pump one of them re-mans until your you know what drops off and still never get it pumped up solid like one should be, they are a waste of time.

When/if you get the new one be sure and follow the bench bleed instructions included with it exactly, if you don't and bench bleed it old style, use of fittings short hose style it won't work. Take your time, it will take several minutes to pump it up removing all the air, do so by hand before putting it on the truck.

If not sure how and the directions are missing be sure and ask before attempting the bench bleed process.


The master on there should be 1-1/16" bore minimum, I'd get the 1-1/8" bore for better results.

The booster might be a issue but only in the rod adjustment, even if slightly undersized it wouldn't effect pedal travel if adjusted and working right. Issue you might notice is the need to lean on the pedal hard under heavy load or in a panic stop condition, the pedal would be solid and not at/near the floor.

Booster pushrod should extend 0.980 to 0.995 from the master cylinder matting surface.

 

thats my problem.. i took it out on my road ( old country road with very minimal traffic.. amd got it to 30 mph and hit the brakes like as if a car/deer got in the way and it took WAY too long to slow down with me standing on the brake pedal.. my main concern was that the undersized booster would cause a issue,even tho i was thinking at worst i would have to put a little more force behind the pedal to compensate for the loss of vacuum surface/pressure..

the rears are adjusted as close as i dare to push them.. any more i fear the truck wont move at all lol..

i havnt touched the mc yet.. trying to rack other peoples brains before i spend unneccesary money.. i thought that a mc went bad by tell tale sign of if you pump it you will get pedal but it will bleed off as you hold the pedal

 

When you installed the pads/shoes did you replace or turn the drums and rotors?

Did you/allow time to break in the new linings?

New linings installed in drums/rotors that haven't been turned, they'd take several miles to seat into the surface wear left by the old linings. New shoes in drums that are heavily worn, at or out of service limit won't seat for several hundred miles if even then. The full surface area of lining won't make contact with the drum, a small percentage of it.

Perhaps a fuller list of work done and some history would help, sudden brake issue or did the brakes always act as they do? as in just bought the truck or?

 

As for an answer to the booster question, the f250 booster seemed to be smaller diameter then f150, but the were longer and stuck out further from the firewall.

Your master should be a 1 1/8in- regardless of the truck. The booster and hydraulic system of the f250's are the same, regardless of the GVWR. When you bleed the system, did you bleed the ABS unit on the driver side frame right behind engine cross member? It sounds like air in the system. Maybe a weak booster, maybe a short push rod its hard to tell.

 

drums and rotors are in spec and clean... break in hasnt had time to do.. but i do know that i should have more than half a inch of actually 'grab' pedal..

as far as the bleeding 1st time bled i skipped the abs.. 2nd 3rd and 4th time bled the abs all 5 spots in proper order... the 5th and last time i completely disconnected the abs and make a little jumper to bypass the abs and see if i could build more pressure, and thats where i stand now...

as far as is it ongoing or sudden.. i have had the truck for about 6 months or so, but have been working on bigger issues and now im pushing to get the "smaller" things taken care of... as long as i have owned it and had it running the brakes were like this, old brakes, calipers etc... all the way up to brand new quality brakes hardware calipers wheel cylinders... all flex hoses are solid. everything moves as should just ( as in if you take drum off and easily push the pedal (2nd person) then you can see everything moving as should be... it just acts like i have no ability to build any real pressure

 

Heres a simple test to see if your shoes are adjusted right.

Jack the rear up, place on stands if you want or whatever. Put the truck in neutral, spin the wheel. it should have drag to it and it should only turn maybe 1-1.5 revolutions before coming to a stop due to drag. If it doesnt stop easily your shoes are not adjusted right.

Look at the inspection port on the backing plate if it has one, the sheos should be near touching the drum. Maybe like an eighth or so of an inch.

If your shoes arent adjusted right the front is doing all the stopping, also the pedal will have TONS of travel.

 

Yea that 1/2" of pedal that's the real problem here, smallish booster or not. I can take some measurements of mine and post them if you like, so you can compare for "peace of mind sake".

Unbolt the master from the booster and measure the rod, no need to disconnect the lines, verify it is set up correct if for nothing else just to eliminate it as the possible cause.
Also look up under the dash while someone mashes the pedal down, make sure nothing is broke or worn out in the pedal bracket/pivot area causing excessive travel.

If its found set within spec and does provide "boost" and doesn't leak vacuum, and the pedal assembly is good, I'd replace the master cylinder with a new one.

 

ok so update... before i decided to start dropping money into unneeded parts i decided i would double check the rear brake adjustment....
the right side was tight... as in it was adjusted properly.. minimal drag but enough to tell..
left side ( drivers side ) was EXTREMELY out of whack.. i had to keep turning for a little bit to get anywhere.. but did get it to the point that it is slightly dragging.. felt the pedal and it seems to have helped a little..

couple questions for everybody..

1. how far off the floor is your pedal when you get a hard pedal? ( sitting still idling )

and 2. which way should you be turning the adjusters from that back side to tighten each side... the drivers side i would think it you would grip onto the bottom of the wheel and move it up ( same way both sides right ) to tighten it i had to take it from top of wheel (on adjuster) down towards the ground.. mind you being its on the back side of where the adjuster 'arm' is on the front side.. my understanding of the adjuster is when you hit the brakes.. it pulls the arm up a little by the cable on it.. when you release its going to push the wheel down and djust the brakes tighter.. but how i had to tighten it then if my understanding of how the adjuster works it would be loosening itself instead of tightening

 

https://www.ford-trucks.com/forums/8...djustment.html Have a read here where subford gives the specs for push rod adjustment.
My pedal would be about a good 4in from the floor with pedal pressed at idle.
Because of the angle of the teeth on the star wheel the auto adjuster can only turn the wheel the one way. So you can easily adjust the brakes up but to loosen them you have to lift the auto adjuster lever off the star wheel to turn it back.

 

good deal on the write up there. hafta check it out.. but on the auto adjusters im just wondering if maybe the 2 sides got put in the wrong sides and maybe causeing a issue with it currently and possibly more headaches in the future.. just curious which way exactly they are supposed to turn on each side

 

Yea adjusters are right and left, right and left hand thread and both are rotated the same direction to adjust the shoes.

Sounds like you have two of the same installed, if you had to turn it as you described.

 

thats what im wonderin on.. hafta tear into it when i get a free chance and see what they look like in there... and to update i have about 2 inches of pedal since i did adjust the brakes... still love to get it to what Lazy K says he is getting at 4 inches.. that would be a great peace of mind knowing i have the extra pedal for any time you gotta stand on your brakes lol

 

ok guys i have been playin with it a little more... changed it out to a bigger proper booster... no change... now here is the killer part... if i let it slowly drift under its own weight, i can stop it by pushing the brake pedal no more than a half inch, but if i push on the pedal it will get kinda hard about 1 inch or so off the floor, but i can push it and hold it clean on the floor without much thought... is anyone else thinking it may be a leaking master? also if you unbolt it from the booster there is just the slightest residue of brake fluid... now i am assuming that that is a sure sign that the master is fried??

 

If the back side of the MC is wet or the pedal sinks to the floor under pressure, it's bad.

btw, I use new MC's oonly-never bought a reman'd one that was good.

Ray