is it normal for a f250's brakes to lock up? do yours?

[fordman]

we are checking up on that in the other post of 4 effies. i think htye started doing that on replacements so as to save money on making two different master cylinders. does the 69 have manual or power brakes?

[1971ford]

power

[fordman]

ok let me do some more checking on this. i dont think there is aproblem. but everyone is wanting answers to this question so i'll ask around bfore i put out my opinon on this.

[jengle]

l1k2gadd,

then we looked at the proportion valve and recentered the it (which was stuck and took some force)

weve done the proportioning valve...

Two things come to mind: 1) if the factory proportioning valve took force to center it, then it's likely not moving freely, as it should. I would replace the proportioning valve with a good used one and see what happens. 2) if you still have a problem, I would buy an adjustable aftermarket proportioning valve for the rear brakes. This will allow you to adjust the amount of pressure going to the rear brakes. An inline adjustable prop valve would be fairly easy for you or a mechanic to plumb in--the hardest part would be re-flaring the lines where you cut it.

[1971ford]

well the after we smacked the proportioning valve and it came loose, it the slid freely when we were adjusting it to the center.
i think its all just a matter of getting more braking power to the front discs instead of a majority to the rear drums correct? i think thats what is being stated here.

[ROBT257]

My 72 F250 4x4 with all drums has the dual size resivour too. Smaller to the front. I am experiencing some brake probs too. I use the truck mostly for around the farm use and hardly ever have to brake hard. Didn't know I had a brake problem until went to the gas station a couple of weeks ago. Approached a stop sign a little to quickly and found that the braking was extremely light and spongy. The front resivour was dry. Figured I had a leak. Refilled, bleed, and found no leaks. Though the kicker is.....when bleeding all four corners untill I got fresh fluid and constant flow with a very stiff short stroke pedal....i found that the rear resivour never flushed through. Still have dirty fluid in the rear resivour. Not a brake system expert. LOL not an expert on anything! Found it strange as after tracing the lines they goe down a couple of feet and tie into a device that I assume is a proportion valve. But the front lines go through to the front and the rear lines (from the rear resivour) keep going to the rear. But when braking the fronts dont seem to be performing that well. The rear wheels will lock up if I really get on it. Haven't tore down the front yet to check the shoes and adjusters yet. My main concern now is the small resivour supplying the fluid to all four corners and rear appearing not to do anything.

[Redcap]

also, i dont want to get off the 71's problem, but my 69 has two different sized resevoirs in the MC too, but it has all drums. whats up with that

That is normal.

[fordman]

Approached a stop sign a little to quickly and found that the braking was extremely light and spongy. The front resivour was dry. Figured I had a leak. Refilled, bleed, and found no leaks. Though the kicker is.....when bleeding all four corners untill I got fresh fluid and constant flow with a very stiff short stroke pedal....i found that the rear resivour never flushed through. Still have dirty fluid in the rear resivour. Found it strange as after tracing the lines they goe down a couple of feet and tie into a device that I assume is a proportion valve. But the front lines go through to the front and the rear lines (from the rear resivour) keep going to the rear. But when braking the fronts dont seem to be performing that well. The rear wheels will lock up if I really get on it. Haven't tore down the front yet to check the shoes and adjusters yet. My main concern now is the small resivour supplying the fluid to all four corners and rear appearing not to do anything.

the front resivoir controls the rear brakes. if it was dry that means the rear brakes have been working and have probably worn down the shoes enough to make the wheel cylinder expand with the longer movement of the brakes shoes before they contact the drums. the rear resivoir still being dirty could be that not enough fluid was ran through when they were bled. so it didnt have to be refilled completely when you bled them. the dirty fluid must have floated to the top when you was gettign clean fluid at the fronts. i would siffon out the old fluid and replace it. and re bleed them. the block you seen the brake lines going into is the brake pressure differential valve. it works with the brake perssure differential switch. and truns on the light in your dash when the switch has an embalance in the brake system. your valve must be stuck open or your light bulb is burned out. it should have came on when the fluid ran out of the master. This leads me to think the front brake may not be operating properly also. the front brakes do most of the stopping on a correctly working braking system. i would pull the hubs and see if you can find anything wrong. but if the lines are all going from only the front resivoir then soemthing is wrong. with something some place. they shouldnt be tied together. both of the lines from the master should go to different hook ups on the differential valve and from the valve to front and rear brakes. is the line from the rear resivoir unhooked or blocked off? that woudl explain alot.

[SteveC]

ok this is off the wal thought but do your rear axles seals leak?? even a little can cause the brakes to lock up bad

[AlleyCat]

ok this is off the wal thought but do your rear axles seals leak?? even a little can cause the brakes to lock up bad

It's not really off the wall in fact it's pretty common. Rear wheel lockup on hard braking is one characteristic shared by every bump and dent F250 I ever owned.

[papabug71]

Thats what I was going to mention. If your axle seals or wheel cylinders are leaking, they will lock up every time(drum brakes). My 01 Dodge has a leaky rear wheel cylinder & it will almost make you hit the dash....

[SteveC]

i drove a 2002 Nissan last year that had a brake problem. you would barley touch the brakes and it would lock up the rear wheels and you would about wreck.. this was caused by rear wheel cylinders leaking.

And just cause they dont have any fluid leaking down the back of the tired doesn't mean they aren't leaking.
I would pull the back drums off and see if it looks like in there.

don't go throwing parts at something until your sure what the problem is. You will lose a lot of customers in the mechanic business doing that.

btw my 67 f250 would do that as well i had a leaking drivers side axle seal.

[m-mman]

Causes of REAR brake lock up under "normal" braking situations:

1. WET shoes (gear oil, brake fluid, even water but that usually evaporates)
2. WET shoes (somebody tried to clean off shoes that have gotten wet with gear oil or brake fluid instead of REPLACING THEM)
3. WET shoes - as mentioned above YOU MUST PULL THE DRUMS to check!

Other causes that are much more rare:
4. Out of round drums - were they turned and trued up? Are you sure?
5. Broken parts like springs - but these would be obvious when you pull the drums (see above)

Even more rare:
6. Shoes that have not been arced to the drums. This has been discussed here before. Today nobody ever arcs shoes anymore BUT it can cause problems.
Darts & Valiants in the 70s were especially susceptiable to rear brake lock if the shoes were not arced. Arcing shoes can be 'simulated' by sanding down THE ENDS of the shoes such that when the shoe (off the truck) is held against the drum (that it will actualy be used on) there is a SPACE and ONLY the middle of the asbestos area touches the drum (dont worry it ALL touches the drum under a load)

7. Ya gotta be clear here. A "pressure diferential switch" (makes the light go on) IS NOT(!) a "proportioning valve"!!! There are TWO different parts!!! (on our trucks)

On a disc system there are 3 separate "things". Sometimes they are combined in one device which always confuses people (Vehicles after about 1980?) but back in the 60s & 70s when dics brakes were OPTIONAL they were designed as 3 different parts so that a drum/drum or disc/drum system could be easily configured. (GM commonly combined them into one device so that confuses people too)

1. Pressure diferential switch - all our trucks have one. They are the same disc or drum systems. There was a good explanation given for these (including pictures) in another post, look for it.
BTW - if yours was stuck it means there could be trouble with crud in the brake system elsewhere too.

2. Metering valve - Rarely discussed and totally misunderstood. This device prevents flow of fluid to the FRONT brakes until a specific pressure has been acheived in the entire system. It acts as a timing device to allow the drum W/C and cups to move outward and touch the shoes to the drums at the same time as the pads touch the rotors. (disc systems dont have to move as far)

3. Proportioning valve - This thing LOWERS the pressure to the REAR brakes. This is so that the rears do not lock up under heavy braking and more braking is done by the front wheels.
Now think about it . . . .IF your brake SYSTEM was so full of crud that the pressure differntial switch stuck, is it possible that the proportionig valve is also stuck and crudded up?
Sure, but most likely you have not checked it because you dont know there are two things to check.

All the above is assuming that your truck has/d STOCK ORIGNAL brakes. If somebody added discs the question comes up "Did they correctly make this modification?"
Modifications made by ignorant people with a wrench in their hand and nothing else to do on a weekend always makes troubleshooting difficult.

But to again answer your original problem about locking rear brakes. WET SHOES, wet shoes . . . .


How about the master cylinder? It is very unlikley that this is the source of your problem. On a dual M/C there are TWO separate cups that create the pressure, BUT they are operating in the same size bore so they will create the exact same PRESSURE for both sides of the system. (reason for the metering valve and proportioning valve)

So WHY are there two different size reserviors?
Under normal operating situations disc systems USE more fluid. WE ARE NOT TALKING ABOUT LEAKS HERE.

Wear down a drum/shoe by 1/16 of an inch and the brake fluid level will drop by 1/16 or an inch, MAYBE (probably less)

Wear down a disc/pad 1/16 of an inch and the brake fluid level may drop 1/4 to 1/2 an inch! So in a disc/drum set up the disc side MUST have a bigger reservior or it will keep going 'dry' UNDER NORMAL CONDITIONS.

[TNIceWolf]

On the 69 Ryan........it means you have the wrong MC on it. ALL 4 wheel drum systems that i know of on a bump era truck or older used either a single resevoir or same sized dual resevoir on them....The single resevoir being on much older vehicles. Also you might want to note that the bumpside era trucks werent exactly built with ABS. They tend to lock up either the front or rear brakes according to the weight distribution at the time of application and according to road conditions. They can be pretty darned unpredictable at times.

[Fastbackin1970]

For what it's worth, the MC for front drums is a different part number then the MC with disc's. They look different also.