I still don't fully understand why this worked for me, but I had a similar issue and it turned out to be the brake booster. I know when the booster goes it's supposed to make the pedal rock hard, but I had the reverse problem. I did a new master cylinder and booster at the same time, bled everything properly, and still had a lot of squishiness and the pedal traveling almost to the floor. After checking everything else a million times, I swapped out the booster for another new one since I was hearing the similar hissing sound from before...cleared everything right up. Brakes were nice and tight and could stop on a dime.

Hope this helps and good luck!

 

You have air or bad rubber brake lines ...

-Enjoy
fh : )_~

 

Removed the rear line from the MC and put a plug in it. Still the same. Hard before starting engine and then start it up and it goes to the floor. 5 pumps and no build up at all.

 

When I had the faulty new brake booster I put on, it was the same thing. Brakes were rock hard when the car wasn't running, start it up and the pedal was super squishy. It must be something about the vacuum running to a faulty booster. I know you said you replaced it, but it could be a faulty part. Do you hear any hissing as you press the pedal?

 

The booster was changed because of the problems I am having now and it didn't change with the new one. I believe it was working fine before I changed all of these parts. There is what sounds like a squeaky pivot point going down and then something else when I let it come back up. Maybe sounds like air sucking sound.

 

Rear brakes not adjusted correctly is one thing to eliminate if work was performed on the rear axle. Yes they are self-adjusting, but the initial adjustment must be done manually or they will never self-adjust. Ever. The reason is they only make fine adjustment (one click) at a time, consequently the shoes must be initially tight up against the drum in the first place for the mechanism to get enough bite to work from there. The pedal will be low and spongy, almost like air in the system. It doesn't take much either.

Sometimes you'll pull a drum on a new old truck and find absolutely no wear on the shoes at all after golly knows how many years, because they were never installed right to begin with and the self-adjusters weren't.

Run them up all the way tight as far as you can, the wheel won't turn, and then depress the brake several times to take all the mechanical slop out. Incidentally this is a good point to bleed them while ran up tight. Then finally back the self adjusters off and spin wheel until a light drag or scraping is heard. Take a test drive and check again.

 

Make sure the hose to the brake booster is clear. An obstruction or a collapsing hose will cause problems.

 

Yup. That is clear. I removed it with the engine running and the engine didn't like that. Massive vacuum leak. There is a ware spot on the line so I wanted to see if it was a problem. With engine running, flexed the line to spread open the ware spot. No reaction from the engine. Would have expected the spot to be sucking air and it didn't so I tried this as well. The line is also solid and shows no sigh of collapse. Thank you for your suggestion. I appreciate any help.

 

The rubber brake lines on the front looked like a tree shedding its bark so I changed them when I changed the calipers. The one that goes from the frame to the rear axle looks fine. No marks, no oil stains and not shedding like the front. Truck is fairly clean and there is no oil on the top of the axle. Showing signs of seeping from the differential gasket so that will be done soon but that is on the rear side and the brake connections are on the front. Tore up the rear yoke and U-joint a few months back so I checked the lines for any possible damage from flying derbies and found no problems. Thank you for your suggestion and help.

 

Yabut "looks" on the outside don't mean anything. (That's true with people btw) Jus' Sayin'.

 

Amen! I agree ...

-Enjoy
fh : )_~

 

Yes the rears were completely replaced. Everything including all springs, adjusters. Got a kit to replace everything. If it goes between the backing plate and the drum, with the exception of the E-brake lever, it was replaced. I believe they are adjusted very close. Drove the truck around the block and backed it up it the driveway a bunch of times. Put on the E-brake, adjusted them until they didn't want to go any more. Released the E-brake and adjusted them until the stars got difficult to turn. E-brake works much better now. One issue with my truck that I didn't mention in my first post is that there is a 4K lb Lance camper in the bed and it isn't balanced properly. Makes it very difficult to get completely off the ground. It also has a limited slip differential so, turn one set of dual tires and the other ones turns, as well as the driveshaft and transmission. I like your recommendations on the adjusting so I can lift one side, remove tires and put on jack-stand. Then lift the other side and do the same. Maybe even drop the driveshaft for this. I really hate having a 10K lb truck that isn't very easy to stop. I did as JWC 3 suggested and removed the rear line from the MC and plugged it. No difference so, while the rears are probably not adjusted correctly, the real problem isn't in the rear. Thank you for your help and advice. I will be readjusting the rears though and maybe even start the truck and run it in reverse with it off the ground and see if I can get it to fine tune it's self before putting the tires back on.

 

Completely agree.
With the rear line removed and plugged up, the problem is still there. I know the main problem isn't in the rear so, I think I will remove both brake lines and plug them both. If the pedal is hard before starting and then goes to the floor with the engine running, it has to be the new power booster. ????

 

Did you ever solve this??? I’m having the same trouble with my breaks!
New booster, new master cylinder times two, completely rebuilt drums all the way around… Don’t see any signs of leaking… And still have a pedal going straight to the floor....

 

Turns out that Ford issued a notice on this problem for the F350 series in they years around my 1990. They put too small of a Master cylinder on the trucks. I had to have an expert look into this and find it. He put on one with a larger inside cylinder. I think mine was 7/8" and he went to 1 1/8". The largest he could without modifying the mounting holes. The Power boosters on these are so powerful, they override everything. I spent over a month figuring this out. Even had to drop the camper because it wouldn't fit in the shop I took it to and they couldn't lift the 10,000 truck. Now if somebody can tell me how to make the self adjusters work properly, I'd be a very happy camper. Good luck with yours.