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Discussion Starter #1
hello,

I have a 1970 GTO that I have just rebuilt the steering, suspension, and brakes. I put on new wheel cylinders, shoes, drums, lines, and hose on the rear and new lines, rotors, pads, calipers, and hoses on the front. I have also put on a new distribution block, master cylinder and power brake booster. All components are new or remanufactured and are correct for a 1970 GTO (no aftermarket stuff). I have bench bleed the master and bleed the brakes multiple times and when I move the car I have to stand on the pedal to get it to stop (and it will barely stop). I am getting plenty of vacuum to the booster and it seems to be functioning. I am at my wits end why the brakes will not function properly. Any idea what I should do next?

Kevin
 

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hello,

I have a 1970 GTO that I have just rebuilt the steering, suspension, and brakes. I put on new wheel cylinders, shoes, drums, lines, and hose on the rear and new lines, rotors, pads, calipers, and hoses on the front. I have also put on a new distribution block, master cylinder and power brake booster. All components are new or remanufactured and are correct for a 1970 GTO (no aftermarket stuff). I have bench bleed the master and bleed the brakes multiple times and when I move the car I have to stand on the pedal to get it to stop (and it will barely stop). I am getting plenty of vacuum to the booster and it seems to be functioning. I am at my wits end why the brakes will not function properly. Any idea what I should do next?

Kevin


Really need more info. New brake pats as in NOS, and or your original brake parts rebuilt or purchased rebuilt brake parts? Originally power brakes? Front disc and rear drums? What is "good" vacuum? New vacuum valve that goes into the booster to which your vacuum line from the carb attaches? DOT 3/4 brake fluid? Good stream of brake fluid coming from each port on the calipers and rear brake cylinders when you crack them open 1 at a time and your helper presses & holds the brake pedal to the floor - starting at the rear passenger side wheel cylinder, rear drivers side, right front, then left front caliper? I'd do this twice to confirm a good strong squirt and ensure any air is purged. Car should be level when doing this. Check master cylinder level often so it does not go dry on you.

Rear brake shoes were adjusted to slightly drag on the drums? Primary & Secondary shoes were correctly placed in position and not reversed? Might pull the drums and have someone gently apply the brakes to check movement of the wheel cylinders pushing outward on the shoes. Need not go much, just enough to verify they are getting pressure and working. Could have easily cocked the pin that goes into the brake cylinder and activates the shoes. There are also a number of other small parts which can easily have fallen off or look like they were correct, but were not.

Did you verify that the brake shoes are fully seated into the drum's arc? Could be a misfit and only an edge (top or bottom) is actually making contact with the drum.

Have you checked for any binding of the disc pads on the pins which hold them? Can you spin the wheel and then have a helper hit the brakes to see how sudden they stop?

Hard pedal pressure could be incorrect brake pedal rod attachment at the brake swing pedal under the dash if you went from manual brakes to power.

Incorrect or improperly adjusted brake booster rod that comes out of the booster and goes into the master cylinder.

Incorrect master cylinder bore size, ie larger than stock. Just had that experience on a 1948 1 1/2 ton International truck. Someone swapped in a replacement, and more readily available, master from a 2 1/2 ton. Bore dia for 2 1/2 ton was 1 3/4" versus the stock 1 1/4" dia. You might not think it would be a problem, but it is. Pedal required 2 feet to stand on it to get it to apply. Larger than stock application will move more volume with less pedal travel, but requires more pedal pressure. Smaller than stock will move less volume and require less pedal effort - as long as you don't run out of pedal stroke needed to move the volume of brake fluid to reach all brake cylinders.

Metallic brake disc pads/shoes which can be too hard and require a lot of heat to get them to work - so it seems like you need a lot of pedal pressure and they don't stop well. I prefer the cheapo and most inexpensive line of brake pads/shoes as they stop better for general driving and won't eat up the rotors & drums. If you feel comfortable with it and have a safe wide open space where you can hit the brakes and do some emergency stops and get the brakes really hot and if they improve, could be the pads/shoes. In lieu of that, I might safely put the car up on jack stands, pointing away from people/objects, and run the engine/wheels up in gear and then do some braking/stops to get them hot and see if you notice any change.

Defective distribution block. I recall a member here having that problem with a brand new one.

Just a few things that come initially to mind. :thumbsup:
 

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Discussion Starter #3
The car is factory front disc / rear drum with power brakes. I replaced the master cylinder and booster with new ones from Year One. The distribution block is a new one from Inline Tube. The drums, shoes, wheel cylinders, and rotors are new ac delco. The calipers are remanufactured from Napa and the pads are Bosch. The pads move fine in the calipers. the rear shoes were adjusted to the new drums. I bled the brakes 3 times starting with the farthest away and then took the master back off and re bench bled it think there must still be air trapped in it, I then bled the brakes one more time. The booster came with a new vacuum valve and I pulled the hose off while running and it was sucking like crazy. The only part I have not replaced is the hold off valve, think that could be the problem?
 

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The car is factory front disc / rear drum with power brakes. I replaced the master cylinder and booster with new ones from Year One. The distribution block is a new one from Inline Tube. The drums, shoes, wheel cylinders, and rotors are new ac delco. The calipers are remanufactured from Napa and the pads are Bosch. The pads move fine in the calipers. the rear shoes were adjusted to the new drums. I bled the brakes 3 times starting with the farthest away and then took the master back off and re bench bled it think there must still be air trapped in it, I then bled the brakes one more time. The booster came with a new vacuum valve and I pulled the hose off while running and it was sucking like crazy. The only part I have not replaced is the hold off valve, think that could be the problem?
Have you replaced the rubber lines?

It is possible. I am not sure if all disc brake cars got the hold off valve or not. You could replace it just to be sure and take it out of the equation as possible causes. The Bosch pads may be of the harder metallic compound. You would have to verify this.

I am no expert on brakes, but there seems to be a distribution block and/or a proportioning valve for 1970. Here is the proportioning block: https://www.opgi.com/gto/1970/brake-systems/proportion-valves/CH26775/

Here is a brake distribution block: https://www.opgi.com/gto/CH26981/

Here is a rear hold-off valve for the 1970 GTO which I would think goes with the distribution block from Inline Tube https://www.ebay.com/itm/70-GTO-Judge-GP-Factory-Disc-Brake-Rear-Hold-Off-Proportioning-Brass-Valve-NOS/200877678510?fits=Year:1970|Model:GTO&hash=item2ec53e1fae:g:iEsAAOSwZ2laoY-f

And there is also a tool to make sure your proportioning valve style distribution block is centered: https://www.opgi.com/gto/1970/brake-systems/proportion-valves/CH31499/

So it appears there are two different ways the brake pressure proportioning was set-up on the 1970 GTO disc/drum cars.
 

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I had this problem on my 56' Bel Air and was chasing ghosts for weeks....Long story short, the valve on the brake booster was defective and bleeding vacuum. So while my car showed great vacuum at the booster and would hold vacuum, when I drove the car I had no power assist.

I pulled the check valve and tested it....Sure enough it was defective. IIRC the part cost < $5 at NAPA or parts store. Replaced with a new valve, and everything worked perfectly.

I'd bet a Corona your valve is bad :).

Chris
 

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I had this problem on my 56' Bel Air and was chasing ghosts for weeks....Long story short, the valve on the brake booster was defective and bleeding vacuum. So while my car showed great vacuum at the booster and would hold vacuum, when I drove the car I had no power assist.

I pulled the check valve and tested it....Sure enough it was defective. IIRC the part cost < $5 at NAPA or parts store. Replaced with a new valve, and everything worked perfectly.

I'd bet a Corona your valve is bad :).

Chris
I put a new check valve in today, Chris when do you want your Corona!

Thank you so much, I was at the end of my rope on this!
 

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Hooray! Success. :thumbsup: This will make a nice archive piece for others that will list things to check...............and the solution. :yesnod:
 
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