Technical Strange brake issue

Have good pedal when the vacuum pump isn't running, it's a separate unit from the engine, kicks on at 16lbs and off at 22lbs. As the video shows good pedal without the key turned to the on position. To the floor with it in the on position. Replaced both the booster and master. The brake idiot light is not hooked up to the combination valve. Have never run into this problem and have bleed the brakes numerous times and no air, and the master was bench bleed. Any ideas ???

 

You might want to check the master to booster link for proper adjustment.

 

Checked with a vernier caliper before installing the second master, good to go. I also took out almost all free-play in the pedal to booster. I have a solid pedal until I turn the key to the on position and the 12 volt vacuum pump kicks on and then to the floor it goes. Yes I have checked and the pump is pulling vacuum.
When I bleed the brakes I have good fluid to all 4 corners so I don't think the valve in the combination valve is off center. I do have a centering tool coming to double check that. Pretty much at a loss at this point.

 

If the pedal is not allowing the booster to fully return to its resting position, that will happen. The act of depressing the pedal opens the valve in the booster which is what allows the vacuum to pull the pedal toward the floor.

That is assuming you did not get two bad boosters in a row, something that I have less confidence in every day. Quality control everywhere isn't what it used to be.

 

Will the brakes hold with the pedal all the way to the floor? Is this a vehicle that this combination originally worked okay or have you changed a lot? I'm guessing the latter. I'm thinking you may have a combination of parts that don't fit together. What's the whole story?

 

Car was manual brakes when I bought it. The master / booster / combination valve, bracket and short lines were bought together as a unit. Car has 4 wheel disc and the kit is set-up for 4 wheel disc. Wish it was mismatched parts. Came from Summit Racing,
Right Stuff Detailing J81315672 - Right Stuff Detailing Master Cylinder and Booster Assemblies. I have brakes that will hold , but pedal is very low.

 

Why the vacuum pump ?

Why the small booster ?

Duel or single diaphragm?

Have you run this set up before or all new build ?

Can you disconnect the booster vacuum and run just manual brakes ? If so how do they feel ?

When you bleed the brakes are you getting decent pressure from the bleeders or just a dribble ?

Stock pedal ? Modified ? Aftermarket ?

Check valve in the vacuum line ?

sound like a failed booster or the booster is staying applied or “ on” all the time .

 

4 wheel disc ?
Correct master ?
No drag or metric calipers ?

 

I still have 1/8"-3/16" free play with the pedal and had a friend help me check things out when the master was off.

 

Too much cam, not enough vacuum made by the engine, small booster to clear the engine, only 1/4" between the valve cover and booster, dual diaphragm, all new set and parts, Have not tried to run it manual yet, but very tempted, 4-5 pumps on each corner equals about 3-4 ounces, Have also power bleed them with good fluid at each corner with 20 pounds pressure in the pot, stock pedal, in the correct hole for power, has two holes, check valve has been checked replaced and put back in, can let the vacuum pump, pump the booster and it holds pressure for at least 5 minutes, didn't test it longer, 4 wheel disc, metric calipers. Master is 1" bore.

 

Better thee than me.
I hope you get some help figuring it out.

 

Original booster was 8" single diaphragm, is now 7" dual diaphragm due to clearance issues with the valve cover.

 

I'm referring to slack at the pedal, not between the booster and mc. Take the rod loose from the pedal and power the pump. If it still goes to the floor the booster is bad. If it doesn't, you need to shorten the rod to the pedal, or make sure the pedal is fully returning all the way.

 

I can physically see the booster returning all the way back, I also adjusted it and took the pin out of the pedal connecting the rod to the pedal, and there is about 3/16" free play before the hole for the pin lines up with the hole for the rod. Anymore free play and the brake lights stay on.

 

My bet is air are you sure the disk brake kits put the bleeders at the very top? Was it disk with the manual system?

 

My luck is two bad boosters in a row. This may very well be a manual set-up before the weekend, I have an adjustable prop. valve coming in tomorrow for the rear brakes
. The master is a deep hole that comes with a spacer to make it a shallow hole, so I already have a manual master at the ready.

 

All four bleeder are at the 12 o'clock position and I have power bleed them 3 times, zero air.

 

Does the pedal have a good return spring?

 

Factory set-up and returns well. Appreciate the help...

 

Brake light switch holding down the pedal?

 

Bad brake hose you might have someone watch the hoses when you apply the brakes

 

I think there’s too much travel in the pedal without the booster. Then the booster kicks in and you hit floor.

May want to go up a size on the M/C bore to get less pedal travel.

 

I was thinking the same thing. Was the original setup manual brakes? And, was linkage reused with the booster? If so, you probably don't have the correct linkage ratio. Remember how much difference there is between the pedal heights of factory power and manual brakes.

 

The car was originally manual brakes. Now power with all new linkage and the push rod is in the lower hole, where it should be for power brakes. That's why the mounting brackets for the booster put it at an angle to get it to line up with the lower hole. The upper hole is where the manual push rod would go. That being said, the lower hole is further from the pivot point and it would indeed have more travel, that's the way GM designed it and why the pedal has two holes on it.

 

You pedal seems low in your video

if you disconnect the brake pedal and then press on the booster with a crew driver or something is it firm ?

still think it’s staying applied


Those 7” boosters do leave a lot to be desired, I’ve heard people having a bunch of issues with them

 

I'll check that today among other things.

 

Could the valve in the Combination Valve .. think there's a problem with the front brakes due to the fact that the one front port is plugged ? Just a thought.. you never know.. typically the lines run to separate calipers from the Combination Valve. I just kicked a guy and his vehicle out of my shop for not taking his brakes seriously. We were doing something similar to what your working on and I was trying to discuss options when he went to the parts store and was taking advice from the counter person. The two determined that I was complicating matters. I was thinking Safety and function. So when I found out he was convinced to go on the cheap I was done right there. Let the counter guy help you reassemble the motherfukkrr. Made him call a tow truck and took photos of it being tow out of here. Hopefully I have documented this good enough to cover my ASS.

 

I checked with the manufacturer of the combination valve to see if it was okay to block one of the front ports before doing it, they said it was perfectly acceptable and would have no effect on the valve. I had wondered the same thing and why I got a hold of them before doing it. The stock front line routing was a single line down to a tee on the frame, I simply replicated that with the new lines. The car is a slightly OT 1966 Bel Air wagon.

 

I’m a bit late to the party, first get some brake hose clamps, or locking pliers, vise grips, but protect the hose with a bit of cardboard, clamp off all hoses, as close to the body end as possible. Now check your pedal without the vacuum pump, it should be as hard as a rock and very little movement, just the free play.

Next turn the vacuum pump on, does it sink a bit? Or go to the floor?

If it goes to the floor, you have problems, booster, master cylinder internally bypassing, or something really weird

If it just dropped a bit, remove the clamps one at a time, get someone to push the pedal, vacuum pump on, while you look at each wheel in turn, check for hoses ballooning, calipers moving a lot. If okay, check for brake pads fitted wrong, jamming in the housing, calipers not sliding on the guides. The caliper pistons maybe traveling to far, e.g taking to much pedal stroke.

Good luck

 

Just a silly little question, does your brake pedal have two holes for the M/C rod? A power brake position and a manual brake position? Some do. Could it be a pedal position/ratio thing?