Power Brake Problem - Could use some help

You really need to know what the hydraulic pressure is at the calipers and wheel cylinders. Then you need to know what pressure they were designed for. Anything else is a guess.

I have a kit from Stainless Steel Brakes that is available at almost every speed catalog retailer. Here is a link to Jegs.
http://www.jegs.com/i/Stainless+Steel+Brakes/884/A1704/10002/-1

 

Dump the combination valve and us a adjustable porportioning valve and a 2 pound residual valve for the disk brakes and a 10 pound residual valve for the drum brakes. I have heard of a lot of people the use combination valves that have brake problems.

 

I think he said this was a tri-five , firewall mounted master, no need for residual valves, 'bout the only thing you haven't eliminated is the combo valve, but are you sure on that pedal ratio, seems awfully "short" !? dave

 

What he said. With out the information provided by this or a simular set up
you are working in the dark.

 

Pivot point to push rod = 4" (Also another hole at 5")
Pivot point to center of brake pedal = 16"

The rear drum setup looks all stock for 56

The front disc's look to be standard Chevy single piston sliding caliper.

We have decided to remove the porportioning valve and install an adjustable valve on the rear. I need to order some fittings.

 

I had a 76 F100 with a disc and drum setup from the factory. When I got it, it too was an average to slow stopper with occassional rear wheel lock-up in an emergency stop. I replaced the rear wheel cylinders with smaller diameters and that cured the lock-up but still slow stopping. Can't remember why, but I had to replace the proportioning valve for some reason and then instantly had very good brakes. The faulty unit was apparently restricting pressure to the discs and with me standing on the pedal to stop it was locking up the rear drums, it also had a firm pedal then too. Good luck GB.

 

#3 answer: The switch is open when the differential valve is centered, IOW's both front and back brakes are getting proper hydraulic pressure. If one of the systems fail, the valve will drift to the bad side and the switch will then conduct.

Taking this further, it sounds very much like the differential valve is getting a large enough differential in front and back brake hydraulic pressures. Meaning that perhaps one of the systems is not working at all. They used to sell a little plug that fit in place of the switch and held the valve centered while bleeding the brakes. If you could get one of these and install, then test brakes, it might point you in the right direction. An alternative would be to bypass the valve all together and test.

 

OK.... I got this car back to work on again and here's what I did.
Bought a brake pressure gauge and tested pressures at the outlet ports of the proportioning block and also at the master cylinder with these results:

Front Brakes - Out of the P Valve: Engine off - 600 PSI / engine on - 1200 psi
Front Brakes - Out of the Master Cyl: Engine off - 600 PSI / engine on - 1200 psi
Rear Brakes - Out of the P Valve: Engine off - 400 PSI / engine on - 800 psi
Rear Brakes - Out of the Master Cyl: Engine off - 600 PSI / engine on - 1200 psi

So I eliminated the P Valve. The front brakes are Teed to the front port in the master and the rear brake line is connected direct to the rear port on the master.

Road test showed marginal improvement.

Removed the front rotors and had them turned and replaced the pads with Ray Best Semi Metallic pads

Road test showed marginal improvement.

So I am still not there. The only thing left that I have not replaced is the booster, which passes all 3 of the tests.

The other question I have is what pressure should I be seeing from a power brake system? Is 1200 psi right?

 

Lots of good stuff here. I am going through the same thing with my truck with 4 wheel disc. Like this guy I swapped booster and M/C. Getting better but just not there yet. Keep plugging away and I will cross my fingers for you. Thanks for the info, Keith

 

With 4 or 5 hard stops from about 35 mph, are the front rotors and rear drums hot?

 

Everything is hot here. It's Texas and 103 degrees. LOL

Seriously, I'll have to run a test and try to get some temp readings.

 

Cheap linings fade fast. Use good lining. Sintered iron or ceramic.

 

I am going to hop in here and offer my two cents. You do not have a booster problem. Boosters do just what they say they do boost pedal pressure and prevent you from having to apply as much leg pressure. Second you do not have a proportioning valve. I know everyone calls it such. What you have is a distribution block. Inside this block is a plunger that moves from front to rear. Directly under the plastic plug you see is a detent groove in the sliding plunger. Remember the two wires that are always attached?? They go to the dash light that reads BRAKE or it should say STOP STUPID. If you lose either the front or back brake pressure the remaining pressure will push the plunger to one side and it will raise the plunger closing the circuit making the warning light come on. The reason you have this is on a single piston unit if you lost brakes well stomp a hole in the floor and drag your feet. They went to the double system in the mid to late 50's and by the early 60's everything had a dual system.
Now not wanting to cause a brew-ha-ha. Size of brake line has nothing to do with pressure. If you put 60# in a 6 inch pipe (with no air trapped) you get 60# at the other end. If you put the same 60# on a one inch pipe you get 60#. What you get different is if you have a ram in each the 1 inch one will move farther than the 6 inch for the same volume of fluid input. What changes with brake lines is velocity of fluid movement. Remember the master cylinder piston size is the determining factor here.
Now these systems come from the factory calibrated out to work properly. They do not need a proportioning valve as such. But when we, the car nuts, get involved we have problems. On the front you have disc which has a big piston but needs little fluid volume to put pressure on the disc because the pads float on the rotor. Now in the back you have drum needing a lot of fluid to move the pistons to spread the shoes. Difference in the two is as the shoes spread there is a mechanical factor that as the shoes catch the drum this also helps spread them applying pressure and since you have this very big piston at the drum you need plenty of fluid to move the shoes. At the front you have this big piston needing very small amounts of fluid to make contact. Disc rely solely on pressure applied to stop the rotor.
Here is what I suggest you do. Go to a 1 1/8 piston caliper you may even have to go to a 1 3/16 bore. Why you need this fluid volume is to activate the rear shoes. The 1 inch you have now should be adequate to make the disc work. By going to the larger bore you will have to apply more leg pressure on the disc go get them to work but wait you have a BOOSTER. Get a true proportioning valve. This will install in the rear line and allow you adjust the rear pressure versus the front.
Next you need to unscrew the plastic plug in you brake block to make sure you have not already pushed the plunger to one side if you have you only have half your braking ability because when it moves one way or the other it blocks the fluid flow. Unscrew the plug and have someone apply brake pressure and as you do it should move the plunger to the center. Screw the plunger back in. You will lose no fluid unless it is screwed up. Wire you in a light on the dash using the plunger to operate it. Work with the new prop. valve as per directions. While adjusting if the light comes on reset it. When all is working as it should the car WILL stop and the light will not come on.

Just my lengthy 2 cents take them or leave them!

 

Take the distribution block out of the system, (at least temporarly) to eliminate it as suspect. On two different occasions in the past three years the distribution block was the culprit.
Your master cylinder should have the residual check valves already built into it.
Wait until you get a good pedal and get functional brakes before trying to fine tune them. You might find out you don't need to spend the money on the adjustable proportioning valve.

 

As I reported in my last post, the distribution block or proportioning valve is no longer in the system. The front disc calipers are teed direct to the front port of the master and the rear line is direct to the rear port of the master. Theoretically, the drums should come on strong and possibly lock up, but they do not.

 

I think you mean a 1-1/8 Master. Although I am looking for anything that will fix this problem please note that the aftermarket brake booster master cylinders packages come with a 1" master and a 7" dual diaphragm booster. I have a friend with (3) Tri 5 chevys. 2 have a single diaphragm booster and the other has a dual diaphragm booster. All have 1" masters and the same GM metric calipers and drums as this 56. They all stop fine.

As I said in my last post, i have removed the distribution block (porportioning valve) completely for testing purposes. The front disc calipers are teed direct to the front port of the master and the rear line is direct to the rear port of the master. Theoretically, the drums should come on strong and possibly lock up, but they do not.

 

Rocco, I bought a 67 Impalla SS with 60 K original miles on it 4 wheel drum brakes it has 1 1/8 master bore on it That suprised me. I built a car for a guy, 48 Ford would not stop same as yours. I went to a 8 IN Booster and a 1 1/8 bore master and it will throw you through the winshield. I've always used the 1 inch but I just wasn't happy with the set up hence the reason for the 1 1/8 keep us posted Thanks

 

When converting a single to a dual master, I always size it to a comparable dual-master donor car. A '67 or so Impala master would be what I'd shoot for. Better too much than too little bore size.

Bob

 

If I read correctly, you said you have a 4 to 1 pedal ratio? Theres your problem. You need more leverage. I would aim for a 7 to 1 ratio, basically doubling your pedal arm length.

Recheck your measurements first.

 

'66-'82 Corvettes came with 1 1/8 bore for power brakes and 1" bore for manual brakes.
These are the cyl most places offer when they market "Corvette style master".

They had 4 wheel disc brakes.

'67-'69 Camaros had the same setup with 1 1/8 for power and 1" for manual.

Manual was 4 drums. Power was front disc/rear drum.

Draw your own conclusions. I tend to go with what the factory did. They paid their engineers to figure this stuff out!

My roadster has 4 wheel Corvette disc brakes and a 1 1/8 master with no booster.
Stops on a dime but needs a strong leg! I used what I had.

 

I will add this to the mix. You are dealing with hydraulics and the best analogy I can come up with is something we are all familiar with, floor jacks. You can take a jack and lift a car but you have to pump the darn thing a lot, you are forcing a very small amount of fluid into a larger area. Ever watch the NASCAR races?? Notice their jacks? One pump and the car is up, takes a lot more pressure on the handle because they are forcing a lot more fluid into the cylinder. That is why you see the fat guys on the jack.
I agree with everyone the 1 inch bore should be enough to make the thing work but like I said you need more fluid to work the back and you need less to work the front.
Try a 7/8 bore and you will see the front grab quickly without a lot of pressure.
On the back be darn sure you have the shoes adjusted properly. Take a brake spoon and run the shoes up until they LOCK the wheel. Now back off approximately SEVEN notches. At this point you should be able to turn the wheel and hear a slight drag. IF the back is not adjusted properly you will never stop the car and you will never get the front to grab. On the pedal issue you may need more leverage but sometimes going to a longer pedal is not feasible for one reason or the other so you will have to resort to changing the input to the cylinders.
If your car is taking a tremendous amount of leg pressure you may have to try blocking off the rear port and seeing if it stops then blocking the front port and seeing if that makes it stop. From what you have done and that is a LOT I do not think you have parts problems. You are going to find it is something so simple and easy you just have to isolate it. Try seeing (and you may have already) which end is working.
I know this, disc do not take a lot of fluid to work but like on the jack you are pushing fluid from a small piston into a much larger piston to generate the pressure you need. It is much easier to push a small amount (smaller bore on the MC) into the disc and get good pressure, just like a floor jack. The larger the piston (master cylinder) the more pressure you will have to apply to achieve braking.
I have a 37 Chevy sedan, way back I installed a Firebird front clip when that was the hot ticket. I used a 10 bolt GM in the rear (drum). Could not get the thing to stop using the original size Master Cylinder. Required pedal pressure was stupid. So I took a booster and master cylinder from a Courier (7/8). Stopped on a dime but required more pedal movement than I liked. I was not satisfied with the setup so I went back and scrounged up the Firebird. Guess what, about half way back they had a unit in the brake line that looked like a ball. (I have never taken it apart so I do not know how it works but I figured they had it in there for a reason.) So I cut my line installed the item (figured it was a residual valve or proportioning valve). Installed the original master cylinder, used the same Courier booster and the thing worked like a top. Cut pedal travel back down to where it should have been.
Oh yea the 7/8 MC did work even after installing the valve but it worked better on pedal travel with the stock MC.

 

Try adjusting out the rod between the booster and the pedal. I had the same problem on a '56 F100 with a disc kit on it. I chased my tail forever trying to solve it, and that was the last thing I thought to try. I had set the pedal to where it looked good, but it turns out it was barely engaging the m/c. Screwed the rod out another 3/8" or so, and on the next test drive I almost broke my nose on the steering wheel, it stopped so quick.

 

Ya never know, always explore the "free" fixes first. Be a hoot if that was it.

Bob

 

I did adjust the rod. Right now, you can barely get the pin in with the pedal all the way up.

 

Well today I borrowed a new booster & master cylinder kit by Master Power Brakes. Kit # BM1746 in their catalog. 7" Dual Diaphragm Booster and 1" Master Cylinder and installed it.

There was no difference in getting this car to stop.

I pulled the rear wheels and verified that the shoes are expanding. I set the adjusters so that the wheels are slightly dragging. Still no difference.

During bleading, I also verified that the calipers are clamping.

A short test drive indicated that none of the things I have done so far have made much of a difference.

So to recap:
Verified 18 inches of vacuum to the booster (vacuum line connected direct to the intake)
Changed Booster
Changed Master Cylinder
Set the rod distance between the booster and master
Set the push rod length between the booster and the foot pedal
Verified that the push rod is straight in line with the booster throughout the travel
Removed the proportioning block
Turned the Rotars & installed new pads
Adjusted the rear brakes

When pulling back into my shop at about 5 MPH I literally stood on the brakes as hard as I could and the car rolled about 6 to 8 feet before stopping.

There is no spongeness and the pedal freeplay is about 1". Total pedal travel is about 3" and it is rock hard at the bottom. There is about 2"-3" between the pedal and the floor at full braking and I have verified with the push rod disconnected that the pedal can full swing all the way to the floor without any binding.

With a really hard push I can actually see the stainless and the flexible lines move from the pressure.

I understand how increasing the diameter of the master or changing the pedal ratio from 4:1 to 6:1 can improve the effort, but I am probably putting 300 lbs of force on the pedal and this car will barely stop.

My friend & I appreciate all the responses and suggestions. Hate to keep throwing parts at this and not getting anywhere. I just don't know where to go at this point.

 

Maybe, just maybe, switch the ports on the master cylinder

 

So a long time ago on a model A, I had a similar problem. It was in fact the rod adjustment from the pedal to the master. I had to have the plunger on the master depressed about an 1/8" to get everything squared away. If you go too far you'll know because even though there is no drag on the rotors in a static position, once going down the road the system will build up heat and pressure and the car will want to stop itself without braking. I would keep lengthening the rod and or adjusting the distance between the booster and the master. With parts these days the specs aren't always what they used to be. My $.002 and 1000 post.

 

Myself and BobK both had a problem just like this. Both of us had Master Power units which I consider junk. I discovered, after trouble shooting for over a year, that the boosters are just not adequate. I went to a local junk yard and got one off an Astro van. That was 7 years ago and everything is still working fine. On my advice Bob did the same. He experienced the same results as I did. During my troubleshooting time I even changed from 4 wheel drum to 4 wheel disc, new lines, junction block and new master. The only thing that made a difference was the Astro booster.

 

Rocco I'd take the advice of P Rat with the 1 in master and then go to a 1 1/8 master if it still isn't good enough, I have sold 2 Master Power cpl units I wouldn't use at swap meets > Good luck.

 

You still need more pedal leverage. Go for 7 or 8 to 1.
Your 4:1 isn't cutting it.