Just replaced master cylinder due to soft fadding peddle. Purged the brake lines (don't know if there is a special procedure). Problem is now my rear brakes are stuck on. If I bleed the rear brake proportioning valve?RABS?Weight control duehicky or whatever the damn thing is the brakes release and I can reapply the brake peddle to lock them back up. Seems like this thing is sticking. No ABS light etc. Fluid filled to rim of brake reservoir. This is a 1994 F350 K model motor. Anyone have a part # or name. It would be much appreciated.

 

Oh and one more thing. The plastic duehicky coming off the master cylinder to the rear brake "damn thing" broke upon removal. Any idea of a name, part number on that gismo... It looks like it has multiple holes in it.

 

UPDATE: The plastic duehicky whatchamacallit thingy was the problem. I dropped the reducer that fits into the brake line output from the master cylinder for the rear brake when I replaced the master cylinder. This in turn KO'd the plastic deal. I removed this item and whalaaa. The rear brakes do not stick. Road test shows the brakes are at impending lockup but work good. Now with a new vacuum pump and master cylinder I still have an issue with brake peddle fading. I hooked up a vacuum gauge to the manifold and I get around 21 on the gauge. When brakes are applied the gauge drops to around 15 and is fluttering bad. More applications and it drops to around 10 or lower. With the booster disconnected I can open a vacuum line completely and it will drop to zero; but, immediately rebound. When the booster is hooked up it doesn't rebound for quite a while. Looks like there might be an internal leak on the booster. So a booster is next on the list.

 

According to the local parts guy it is called a check valve.

 

Unfortunately the check valve is unable to be located through Ford or other auto parts. The brakes are working well though. I have snow/ice to play in and empty the brakes work well. I would like to replace it as it would make sense that it was put in there for a reason. Anyone have any idea where to find one. Looking at local auto dismantlers they don't have donor vehicles. I'll keep looking...

 

you just need the plastic check valve for the booster???Grab one from any booster Ford used the same one for years gas and diesel. the part is also available new let me find the part number

 

it is a dorman part number its 80190 and can be found in the help section of most auto part stores that sell dorman motormite products and costs about 8.00.

 

Thanks Ford390gashog. Unfortunately, it is not off of the booster. It is off of the master cylinder. The rear line that comes off the master cylinder and is before the adapter to the brake line which runs down the frame rail to the ABS system? This plastic disc that looks like a frisbee with holes all around the edges. If you have that part # or even the name of the item that would be great!

 

Hi Brad,
By "adapter", I think you're talking about the proportioning valve, correct?
Is the piece you need in the master cylinder, under the proportioning valve?


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^^^^^^ Good post.
Also a bad booster will not cause a low/sinking pedal. Make sure that your rear brakes are properly adjusted up and that all air is out of the system.

 

I just took an old master cyl and removed the proportioning valve. No plastic disk.
I have no idea what you have...
I couldn't figure out how the proportioning valve works, either.
My understanding is that the front disk brakes require more pressure than the rear drums, and that the proportioning valve is supposed to limit the pressure going to the rears, bleeding off the excess. Looking at the valve out of the master cyl, I couldn't see any way for it to bleed off any fluid...

Is your brake pedal firm with the engine off? Every time I have the front brake system open on my 93 F350 (or my 93 F250), when I bleed it I end up with a firm pedal with the engine off. Start the engine and get vacuum to the booster, I get a pedal that sinks pretty low. I have to bleed the bejesus out of the fronts to get it better. What I think happens is that some air gets trapped the the front circuit that is hard to get out, and without the booster I don't get enough pressure to compress it much. With the engine started & the booster working, there's enough pressure for the proportioning valve to bleed off pressure to the rears, enough pressure to compress the air in the front, and therefore the pedal goes down...
I don't know for sure, but if I bleed the fronts, go for a drive on a washboard road to shake things up, bleed the fronts again, do it again, repeat, etc., I eventually get a firm pedal with the booster on.

 

PHY,
Yes, it is "under" the proportioning valve or between the valve and the master cylinder. It seemed to have something to do with braking pressure going to the rears. With the ice/snow last night and the truck empty except for 3-4 inches of snow in the back the brakes work great. Doesn't seem to want to lock up the rears prematurely.
I know what you mean about bleeding the brakes. I've done exactly the same thing, bleeding the brakes, driving, bleeding, driving. I bled enough out of the system that it is all fresh fluid now Maybe someone had the same issue and broke the one that you looked at/removed it or is it from the same year/type of system?
Lazy K
I've got a good peddle now. It seems I had a bad booster as It would not hold vacuum when it was isolated from the rest of the system - strange it really acted up right after applying the brake and "reset itself" after a while.

Well after a new vacuum pump, master cylinder, booster and 6 gallons of fluid it seems to work well. I haven't tried braking with a load and I am curious if that dohicky (that is missing) under the proportioning valve will affect the braking performance.

 

There wasn't anything under the proportioning valve in the new master cyl I put in, either. I have no idea what it is... Are there any holes in the master cyl that would line up with the holes in the plastic disc?

 

Phy
Good question. Didn't think of looking. Not really wanting to pull the line off to check and have to rebleed; but, I might just to satisfy my curiosity. The disc looks like it is supposed to "float" or spin in there - not sure though just a guess.

 

If it's good now, I wouldn't pull it to look. It's such a pain to get it bled out well, I really hate to open it...
Glad it's workin'!