A little puzzled about my braking problem. Not sure if it's air in the lines, or power brake booster, or not enough vacuum.

('56 f-100, 72 Torino front clip with discs in front, drums in back)

They used to work fine but slowly came to this condition.

Brakes are mushy, pedal goes to floor with out much resistance (while engine is running).
With engine off, pedal is firm. But when I have brake pedal pressed and start engine, it sinks straight to the floor.
When driving at 30 mhp and slam on brakes, it slows down, but the brakes don't grab like they should.
'At times' when driving and need to slow down, they act like they want to grab when I lightly press the pedal, causing me to tap the pedal once then apply pressure at a moderate rate to slow down.
For fast stopping, I tap the pedal then apply firm pressure, almost like pumping the pedal no a non-power system. But no way in hell will it lock up the tires.
And when I don't have time to pump the brakes, I've found myself rolling into things (like intersections)

A couple months ago I put in 2 rear wheel cylinders and a master cylinder and bleed all the air out (so I think).
So I'm not sure if it's still got air in the lines way at the top, or not enough vacuum for the booster (I have a 351C with a pretty decent cam)
I did a vacuum pull on the booster and it held, but maybe I need a canister.




Any ideas???????

 

my initial reaction... master cylinder is failing... bypassing internally.

they did work correctly when you replaced rear cylinders?

your booster test is what i have always read...

If, when sitting still, engine running, pump the pedal twice and hold... does the pedal sink slowly to the floor then?

rear brakes adjusted correctly?

 

When replacing a MC you usually have to "bench bleed" it, otherwise it will take you a long time to bleed all the air out of it at the wheels. My first thought is you still have air in the system. After replacing components, I will usually start with the wheel CLOSEST to the MC and bleed back to the furthest. That's opposite most manuals, but I never understood the purpose of pushing all the air thru the longest line? Be sure the MC never goes dry during the bleeding or you'll have to start all over.
I can't see where you have the vaccuum line to the booster hooked up, but be sure it's to the intake manifold, not to the carb. Big cam + big carb = low vaccuum at idle. put a vaccuum gauge on a T in the booster line and see how much vaccuum you are getting. If it's fluctuating or low, you'll likely need a resevior (sp?) tank or an electric vaccuum pump to power the booster.

 

Replacing the rears didn't make a difference.
So I got a new master cylinder, still didn't help. And did bench bleed it. Then bled out even more bubbles awhile after the install. (I thought I got them all out at the bench... Doh!!!) helped a little but not much.

I am pulling vac from the intake.
I don't have a vac guage handy at the time, so I'll have to check that later.
Plus I'll re-check the rear adjustments.

I never understud pushing air through the whole system either.

 

what about the booster ? i had a 72 cutlass w/ mild 350 olds that gave me all sorts of goofy problems and i went through the entire system and replaced it all except for the booster . turns out the diaphragm was bad and leaking a little .

 

I'd rebleed the brakes. Start at the MC by cracking the lines (DON'T get any fluid on any painted surfaces, it's a very effective paint remover.) Then work front to back, don't stop until you get no bubbles. Put a plastic hose on the bleeder and put the end into a clean jar partially full of fluid so the end of the hose is immersed in the fluid. watch the hose for bubbles.
Did you replace the wheel cylinders because of your brake problem or because they were leaking? If they weren't leaking (take off the dust caps and see if there is fluid behind them) then they were not the source of your problems. Did you adjust the brakes properly? Rotate the tire while adjusting them tighter with the brake spoon until the wheel locks. While trying to turn the wheel back the adjuster off until there is still a slight drag on it. Does the emergency brake work?

 

Oh yah, when bleeding front to back, once you reach the furthest wheel, rebleed the other 3 working back to the shortest line. There actually is a reason to do them int hat order, I just do it the other way around when I have a lot of air to get rid of. I don't see a proportioning valve. Is the MC you used one for disk-drum application and does it have a built in proportioning valve, or one remotely located?

 

not that I know of, but it is a stock one for that year and make of Torino.

 

So was the rear end out of the Torino as well? Is that thing just behind the upper A arm part of the brake system? Looks like it could be a proportioning valve. If it is, that may be where your air is hiding.

 

Are those your brake lines running across the booster? You have a local high point in the lines that will be nearly impossible to bleed the air out of. The air has no reason to leave that "trap". The lines need to run continuously uphill to the M/C. If you have loops in the lines, like stock, they need to be horizontal and continuously spiraling upward towards the M/C.

 

I'm with axracer. He said it worked "before replacing" the m/c and rear brakes, but slowly became defective. M/c could still be bad as some venders can have a run of defective rebuilds. Disc/drum, is numbers dummy available to verfy part numbers. Some bleeding systems are very difficult to complete. Some need a pressure bleeder to accomplish bleeding. Axracer talked about correctly adjusting rear brakes, yes!!! Bleed front to back, short to long. I'm also a dot 5 brake fluid fan since corvette days. Anyone tried it in their trucks? Have a great day, chuck

 

Ha ha ha, "numbers dummy" sums it up.
Yes those are the brake lines, and they meet up at that junction.
How do I go about getting the air out of that thing? push in the valve while pedal is pressed?

I'll adjust the rears again tomorrow, I'm pretty sure they're fine.
But then again I do loosen them up at the track so they don't drag when I do my burn out. But I've adjust them since my last time out.

 

I'm not sure what you'd press in, the proportioning valve all works internally.

 

If you figure out what it is report back since I also have the same problem. I have replaced the MC, tested the booster, bled the lines numerous times and I still can't get decent brakes. I'm hoping that sitting all winter did something to help...yeah, right!

 

Some Ford combo valves need the metering valve held open by pushing/pulling on a little pin on the end. I think his brake lines are fine. I have both lines go higher than that and never had a problem bleeding the brakes.