Hi,
I checked the search feature already for this issue and couldn't find anything, so here we go.

I have a 71 f250, with 4 drum brakes. I just got the 557 running and the thing was driving fine and here comes an intersection and i have barely any brakes.

i thought it was the camshaft but after checking it it does suck the pedal down when starting it up, which made me think that i either have air in the lines or some wheels cylinders were bad.

so i replaced all four wheel cylinders, 2 were leaking. replaced a brake hose that was all bent and kinked. I sort of bench bled the master cylinder by running some clear vinyl lines from the outlets back into the reservoir, and pumped the hell out of it. everything seemed alright there, so tonight i bled all 4 corners.

I did the whole right rear, left rear, right front, left front thing, and even did them again backwards. even though the pedal i better than it was before, its still goes to the floor. the push rod on the pedal seems to be adjusted well, theres a bit of backlash to let the MC come all the way back.

so i guess the main question is, do i have a bad master or a bad booster, or some serious air in the lines? How much brake fluid does it take to bleed a system out? If the brake booster were good, would it not get harder with each pump while the engine was off? because it doesn't. Ill test the master by plugging the ports up and seeing if the pedal gets rock hard tomorrow. If it does get hard and stays hard, then i can assume that either there's a ton of air still in the lines or the brake booster is bad right?

some back story. the master cylinder and brake booster are three years old, barely have been run. in fact they sat for 3 years with no service on them or use. the front part of the rez was pretty much empty, i could touch the floor of the rez...that's how low the fluid got. the brakes worked fine before the truck sat for 3 years. thanks in advance for any help,

mike

 

Sounds like you have a leak in the rear system someplace. If the M/C were bad and by-passing the fluid level would not go down as yours is. The front side of the reservoir does the rear system. The booster only assists so it would not have anything to do with your pedal going down to the floor.

 

In my 69, the front is actually servicing the front brakes and all the lines look original. I know it's historically the other way around. New cylinders can also be junk. It's all a matter of what material the cups are really made of. I'm also assuming you didn't use DOT-5 but stayed with DOT-3.

 

Yeah, dot 3

 

There were no outside signs of leakage on my 69 when I went through the front brakes. When I pulled the passenger side I saw where the fluid was going. The whole thing was soaked, but the amount that squirted out was minimal. About 6 drops worth. Doesn't take long to pull the drums. Peace of mind is priceless IMHO. Did you replace the soft lines too? And yes, the brakes should be more difficult to depress with the engine off and the saved shot spent.

 

Yeah, the front right and left rear were leaking, I replaced all four for good measure. How much fluid should I go thru before I know when to stop?

 

In my experience it's when the bubbles stop so 7-10 cycles. I usually do two or three once the bubbles stop for good measure. Rule #1 for old rides: First it must stop. Everything else is secondary. My brake bleeding set is a jar with a hole punched in the lid with a piece of vinyl tubing going in. The other end of the tubing has some rubber hose to adapt to the bleeder nipple.

 

[QUOTE=Rasputin53;13809437]In my experience it's when the bubbles stop so 7-10 cycles. I usually do two or three once the bubbles stop for good measure. Rule #1 for old rides: First it must stop. Everything else is secondary. My brake bleeding set is a jar with a hole punched in the lid with a piece of vinyl tubing going in. The other end of the tubing has some rubber hose to adapt to the bleeder nipple.[/QUOTE

Well I tested the M/C and that checked out okay so I'm left to assume thered a lot of aair in the system. Bleeding it out right now actually. Doin the ol' pump hold bleed tighten method. Gonna be a long cold night.

 

I bled the bubbles out quite a bit last night. Not sure if its all out though, pedal still goes to the floor with no problem. The MC seems fine , plugged the fnt/rear ports and mashed the brake pedal...I was and stayed hard as a rock, so the seals are good. Next step I guess is replace all brake hoses, then do all the pads. After that not sure what to do. Go to a brake guy...let him figure it out.

 

Look at the whole system for leaks. Proportioning valve,metal lines, Metal line along frame,etc. When the prop valve goes bad it will leak up through the switch mounted to the top of it.

 

Have you adjusted the shoes correctly? They need to be close to the drum or you will get tons of travel. Pump the brakes several times and see if the pedal comes up. I adjust the shoes out until they start to drag (not just barely scrape, but actually start to bite) and then back off 4 clicks. Puts the pedal right below the top.

And just a note: The distribution block is what the brake warning light switch plugs into. It's only purpose is to operate that switch and provide the split for the front brake lines. The proportioning valve (cylindrical part) delays the action on the back brakes. They can leak too, but are only present in disc/drum setups. Not used in drum/drum setups. In 70 they were combined into one unit which I believe is where the confusion started.

 

10-4. I'll do a detailed check for leaks tonight

 

If you had leaking wheel cylinders it is likely the Brake Pressure Differential Valve in the distribution block has moved off center and you will only be feeding partial pressure to either the front or rear system. That could cause the pedal to travel farther than it should. I'm not sure if there is a way to reset the valve while the block is still on the vehicle. I removed mine, replaced the seals and then made a bolt with a turned down tip that goes in place of the switch, it holds the valve in place while I bleed the brakes.


Maybe someone will offer additional information or shoot down my idea if it is incorrect.

 

Well I have access to a lathe. What size bolt did you turn down? I'll probably pull that thing tonight to take a look at it if its leaking.

 

The bolt I used on my '67 F350 is a 3/8-24 cap screw.