I did a complete rebuild of my brake system in April. Drums, shoes, wheel cylinders, springs, hoses, pads, rotors, calipers, master cylinder, proportioner valve and flushed all the lines and refilled with synthetic fluid. I have made five trips from Southern California to Idaho hauling home furnishings, one trip to Oregon and one trip to Northern California since overhauling the brakes with everything working great. Now something strange is happening. The brake pedal goes to the floor with very little resistance. It isn’t spongy like there is air in the lines. It just goes to the floor if you push it all the way down.

The truck stops ok but it is really scary with the pedal going down like that. You can feel when the rear brakes come on because there is a little resistance in the pedal when they come on. You can actually lock the rear brakes up and skid the rear tires. If you push the pedal down further the front brakes start coming on and if you push all the way to the floor the truck will nosedive and you will get planted in the windshield.

I wonder if the aftermarket Bronco Graveyard proportioner valve is blowing all the pressure to the rear wheels because it won't do it if you take the vacuum off of the vacuum booster. You have full pedal when the booster is deactivated and all the wheels operate normally with the exception of having to plant both feet on the pedal to stop it.

Does anybody have experience with this happening?

 

it sounds like you have a leaking brake line.
first thing i would do is make sure the master cylinder is full. if not, fill it up. then step on the pedal a few times, and get out of the truck and look for a wet spot under it. this will tell you where the leak is.

 

I appreciate the input, but there are no leaks. The fluid levels stay the same in the master cylinder and the way the break pedal goes to the floor it would have to be an open line to go down that easy. If you push down fast on the pedal there is no resistance. You stomp on the pedal and it hits the stop at the end of its travel so hard it hurts your foot.

If you push down on the pedal real slow you can feel a slight drag when the rear brakes activate and if you keep pushing you can feel another slight drag where the front bakes activate.

I don’t think it is the master cylinder because it is a brand new unit purchased this last April, but I am going to pick another one up today and replace it. I believe it is the aftermarket proportioner valve purchased at Bronco Graveyard. I was hoping someone on here had the same experience and could shed the light on this problem. I hate shot gunning problems, but without help I guess this is my only alternative. The brakes worked really well for almost five months and 10,000 Miles after rebuilding them. Thanks again for your response.

 

i can't see a proportioning valve doing that.
my 04 started acting the same way, rock hard pedal, then next time it would go almost to the floor before stopping. in the past few days it now goes to within 1 inch of the floor before brakes applying. i an replacing the master tomorrow because everything else is new.

 

I bought a rebuilt master cylinder this morning. I am going to put it on in the morning. I just find it hard to believe that a brand new, not rebuilt, master cylinder would only last 5 months. That is why I was exploring different avenues. I just couldn’t believe it was the master cylinder. I will know tomorrow.

 

Well I just replaced the master cylinder, same thing. Sitting still with the engine running the brake pedal will go clear to the stop at the floor. Push the brake pedal down about two inches while driving and the rear brakes lock up and slide. Go down to about two inches from the floor and the front brakes come on. If you slam the pedal to the floor the darn thing will throw you into the windshield. Everything in the brake system is new except the steel lines. It has to be the proportioner valve. I am trying to find the right adapters to install an adjustable proportioner valve.

 

i wonder if maybe the booster is bad?
another thought. what ABS is on your truck, 4 wheel, or just the rear ABS unit on the frame?
i had a chevy come in the shop with the same to the floor brake issue after a blown brake line.
the ABS needed a computerized bleed process to get all the air out of it, and now the pedal is all the way up top again.

 

It's not the booster itself. A bad booster will only make the pedal harder. They can't fail in a way that lets the pedal go to the floor.

If it's the truck in his sig, there's no ABS.

Did you bench bleed the master before installing it?

 

You have to understand that this thing was working fine for four months prior to this happening. Usually air in the lines just gives you a spongy pedal. This thing just goes straight to the floor like a line is completely open. You can actually push the pedal to the stop on the floor with your hand. And yes the master cylinder was bleed before installing it. I just put another master cylinder in this week do to insistence from other people that it had to be the master cylinder. I bench bleed this one as well before installing it and the symptoms are identical to the other new (not rebuilt) master cylinder that I put on in April.

 

This is a 1977 F250. To my knowledge they hadn’t even invented ABS yet. I had a 70 Chevy pickup years ago that the brake light would occasionally go on and the brakes would exhibit about the same symptoms as my 77 is now having. I would take a hammer and pound on the proportioner valve until the light would go out and the brakes would go back to normal. That’s why I think it has to be the proportioner valve.

I had just installed this new aftermarket proportioner valve 5 months ago and heard they were junk. I was just hoping some one on FTE had the same situation happen to them to confirm that this is the problem. I am not going to waste the money on another one so I am planning on putting one of the Adjustable single units on that the guys are putting on racecars. This will eliminate the front brakes going through the proportioner valve.

The problem is the rear brake line has such screwy fitting on it that they don’t even make an adapter with 9/16-18 threads on it to connect the rear line up. I could put a standard nut on the rear line but it would require doing a double flare and I have never been able to successfully do one. The thought of me messing up a brake line is really scary. The only other option would be to find two fittings. One with the 3/8-24 female inverted flare fitting and another one with the 9/16-18 female inverted flare fitting and braze them together.

 

I'm wondering if a vacuum leak has developed in the booster swivel or the line coming from the intake? I'm aware of the test you did with the vacuum line disconnected....

If you still have the old proportioning/metering valve could you take it apart, clean it and re-use it? I'm not familiar with the JBG's valve. Does it have the little pin in front that you have to push in to bleed the front brakes?

I'm presupposing the metering needle is centered 'cause you've stomped on the pedal a few times already. The brake warning light is not on at all, yes?

 

Like an idiot I threw the original unit away. It was really rusted up. I pulled the switch on top of the proportioner out while I was rebuilding the brake system and the whole inside of the valve was rusted. That is why I decided to go with a new unit.

The new unit came with an adaptor that you replaced the switch with while you bleed the brakes. I did that and when this problem cropped up I pulled the switch out and put a small screwdriver in the hole and the pin is centered just like it’s supposed to be. I did that just to make sure it was centered to eliminate the idea that the switch might be bad and not indicating.

If there were a vacuum leak in the booster you would have a hard pedal not one that you can push to the floor with your hand.

 

ok, sorry about that. i just assumed it was a newer truck.
i got 1/4 the pedal back on my 02 be removing the ABS unit. apparently the abs was bad and blocking off the rear brake circuit.
adjusting the pushrod when installing a new master made a slight improvement, but i still get nothing for over 1/2 pedal.
i have a booster from a wrecked truck i am going to try, because i hear something when i step on the pedal.
but since i am 80+% deaf in my left ear and around 50% in the right, i can't pinpoint what i am hearing.

 

I'm aware of that. Just a thought about the line going to the booster.

 

dang it!!! sorry Fast, i just had a major idea come out of my senility.
you have a 77 f250 DISC FRONT AND DRUM REAR, CORRECT?
the major cause for low pedal on drum brake vehicles is out of adjustment brakes. try adjusting the brakes and see what that does for the pedal travel.
jack up the axle and with trans in neutral spin the wheel. tighten the brakes until you either hear them contact the drum or it stops spinning freely. i usually set drum brakes to 1 full turn before stop.