Hello all,
I recently installed new shoes and brake springs on the back of my truck. I drove the truck for approximatly 1 mile and the pedal went all the way to the floor. (good thing I was going slow) I pumped the brakes twice and had a good stiff pedal again and the same thing happened. I went back home and pulled the back tires and brake drums off of the truck thinking I had just blown a wheel cylinder. everything was dry and no leakage anywhere. I pulled the front wheels off and found no leakage there either. I then went over every thing from the master cylinder down and found no leakage anywhere. I am not losing any fluid at all. I bled the brakes out and still had no luck. The brakes are excellent but I have to pump them once or twice now before I get a good stiff pedal and if I don't they will go almost all the way to the floor and the brake indicator will come on. If I pump them and then hold them down they will creep almost all the way to the floor also. I was thinking it is probably the master cylinder bypassing internally until I got talking to a friend of mine who thinks it may be the proportioning valve. The back brakes release totally everytime as I checked this when I had the back end apart the second time. This truck is also equipped with a load sensing valve that does something or another to the brakes. could this be it? Am I right in thinking the master cylinder? I would have had no doubts if my friend hadn't mentioned the proportioning valve. I did not have the system apart to get air in it, and I can't figure out how I would have gotten air into the system with no leaks. I don't want to replace the master cylinder just to find out it isn't the culprit. Anyone have any ideas? Thanks in advance and sorry this post is so long.

Brad Godkin
1986 F350 CC/SRW
6.9liter/C6

 

Sound exactly like what happened to a gal I work with. She went to stop and had to pump the brakes and still had to press it as hard as she could. I first off suspected a vacuum leak or power booster failure. When I went to check it out, I could hear a constant hiss if I pushed the pedal down a little bit (with engine running). The pedal was stiff like normal with the engine off. It was her power brake booster, somehow the diaphram inside decided to fail while she was driving down the road.

 

If the problem just started after you redid the rear brakes
then I would suspect the shoes are out of adjustment. They
are too far away from the drums. This is why you have to pump
brakes. After you pump the shoes out to the drums, the springs
pull them back in again and you have to pump them back out again.

 

i agree with franklin, it could be the shoes not adjusted close enough to the drum.

 

My friend has his rear's adjusted out so he can do brake stands. His fronts hold and work just fine.


But that's just my opinion.

 

I would make sure the brake pads aren't to far away from the drums. You have to remember that a hydraulic system works by changing a small amount of force over a large distance into a large amount of force over a small distance. Make sure the brake pads are resting against the wheel drums. Do this by using the spindel wheel nut adjustment tool on the spindel wheel nut. You should be able to adjust it thru a small port at the base of the wheel drum that is facing the axle. To tighten the pads against the wheel drum I believe you need to turn the spindel wheel down towards the ground. Do this until you can't pull the brake drum off. Let me know what happens!


Diagrahm (as viewed from the back of the truck facing forward on the driver's side)

wheel drum
__
|
|=+ <--bleeder valve
|
|====================== <-- axle
|======================
|
( ) <-- small port for spindel wheel adjustment.
|
__|

Hope this helps!!!

 

AH NUTS! My picture didn't come out exactly like I wanted it to. Just move the margin bars(that represent the wheel) over two spaces and you'll get the picture.

 

I forgot to mention that I adjusted the rear brakes up until I could just barely hear a little friction between the pads and the drums with the brakes off. The auto adjusters should also be working, I put in new arms, cables, and retainers just for good measure when I did the brakes. The reason I am thinking master cylinder is that when I do get a good hard pedal it feels like it is bledding off if I leave my foot on the brakes. I don't know alot about the proportioning valve so I am not sure if it would cause this problem or not. Thanks for all the help though. It is good to have some folks to bounce stuff like this off of.

Brad Godkin
1986 F350 CC/SRW
6.9liter/C6

 

Brad, Rear brake problems are what hooked me up with this forum in the first place. Here are some suggestions. 1st thing to do, check vacuum line to booster and booster itself. 2nd thing to check would be the master cylinder itself. Have you had to add any fluid in the past? Before and after any work that you have performed? I have seen fluid leak from the master into the booster and actually have the fluid burned through the engine via the vacuum system. If you are positive that the system is not leaking and that the master cylinder and booster are good, then my next question would be that have you checked the RABS valve? My primary problem with my truck was the RABS valve. In all my life and working around all size Fords plus Internationals and G.m.c.'s, the RABS valve got me good. I replaced the master cylinder, front pads, rear shoes, rear cylinders, and then total confusion. Front calipers were replaced the year before and there was no leaks. Finally Paul suggested the RABS valve. The pedal would settle and drift. Rear brakes did most of the work. Bingo! RABS valve. Night and day. Unbelievable. Sorry to go on a rant but this thing was killer to me and others. Just a thought. Good luck.

Jakegypsum // Ford fleet, soup to nuts.

 

Brad, you should probably have more tension on the rear shoes than a slight friction ,but if the pedal is creeping to the floor after you pump them up my guess is a master cly. did you bleed the brakes before this happened or back the front pads off and have to reset them alot of times when this happens the master cyl. piston goes beyond where it normally does and picks up dirt or rust and ruins the seal which lets fluid bypass and then no brakes or very little. let me know what happens.
John

 

Hey Jake,
I am going to check the booster in the morning to see if there is any fluid leaking into it. Never thought of that one. I have never had to add fluid at all. My truck is an 86 model F350 CC/SRW with a 6.9 liter diesel, No RABS installed on this old girl (thank God ) Just out of curiousity, what does the weight sensing valve that is hooked to the rear end do on these things anyway? I have seen no reference to it in Haynes or Chiltons manuals and have no experience with them. Thanks for the help.

Brad Godkin
1986 F350 CC/SRW
6.9liter/C6

 

Hey John,
I think you may be right. I adjusted the brakes up pretty tight and they still don't seem to be working any better. I took it to a buddies shop and ran the power bleeder on it and it still doesn't help. same prob. Then I called Pep Boys and found out a new master cyl. is only 19 dollers, think I'll go pick one up in the morning. Thanks everyone, I'll let you know how I make out.

Brad Godkin
1986 F350 CC/SRW
6.9liter/C6

 

The weight sensing valve your are talking about senses the load in the bed of the truck. As load increases this sensor tells the back brakes to brake more/harder to take the load off of the front brakes. On trucks most of the time the front brakes are doing the braking. I am guessing at this but I believe it is a ratio of 70/30. When this sensor kicks in the back brakes really start pulling their weight and not just hangin around at the back taking it easy.

 

lol thanks, I figured it must be something like that but wasn't sure. Thanks for the help everyone. I will let you know how it turns out. it decided to freeze rain and snow last night so I might put off working on it another day. truck only fits about 3/4 of the way in my garage.

Brad Godkin
1986 F350 CC/SRW
6.9liter/C6

 

Hey Everyone,
I pulled apart the back brakes again today and found both auto adjust cables had come unhooked from the adjuster arms. After a little more in depth study I put them back on right and adjusted the brakes up relatively tight(medium friction between brake shoes and drums) work alot better, My master cylinder is shot but I will put that on in a little bit. I think what caused this is I didn't have the brakes adjusted properly as I had thought and I over traveled the master cylinder, scoring the seals. I do not have any external leakage but It is bypassing inside for sure. I pulled the vacum hose off the booster and it is dry also just like it should be. Thanks everyone for your inputs and advise.

Brad Godkin
1986 F350 CC/SRW
6.9liter/C6