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#21
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When I converted a 64 chevy 1 ton (all drum) from a single pot to a dual master cylinder, I added 10 psi residual pressure valve (RPV) from Wilwood to each circuit (the new master had to built in RPVs). 10 psi RPVs keep the springs from pulling the shoes too far back. I think you need them but I don't see how the lack of them would cause the brakes to be grabby.
Also when you say the fronts grab, is it both fronts or does it pull to one side?
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Kent R. |
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#22
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First. Thank you to all of you. I am carefully going over your advice
I've been driving the car around today. I've got a pedal that goes down about half way then the brakes come on quick and strong. Not grabbing after changing the rear shoes. I would like to be able to fix that pedal drop and it will be great. I can say with certainty that my master cylinder is a 1" bore. I measured the new one and the old one and both 1". All of the research I've done shows that the 4 wheel drum cars had a 1" bore, and also., that's what aftermarket lists. Regarding the booster to Mc pushrod, I have just under 1/8" play measured by putting some body putty on the rod, assembling the setup., and measuring the crush height of the clay. I have no fluid loss in the reservoirs, no tracable leaks, all adjusters are set up for a light drag on the wheels. All shoes are also new and of same brand. I took a video of my Mc with the lid off : https://youtube.com/shorts/5uTBwojVVnw?feature=share and right when I press the pedal, there is a 'slight' geyser effect right when the pedal is pushed, and then no effect on release of pedal. Are the 10lb residual pressure valves (either in-master, or wildwood inline inserts) designed to keep the inner cups in the wheel cylinders against the wheel cylinder push rods? I'm wondering if that possible variable in 4 wheels could be causing the pedal drop. There is no "spongy" when the pedal drops halfway, but I am certain the Mc is being engaged. I found a Mc on ebay that looks in very good shape. I am hoping it will have the rpv's in good shape because I would like the original look. Any thoughts? Last edited by Hotrodjohn71; 01-20-2023 at 11:43 PM. |
#23
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Make sure you have all the air bled from the lines (farthest to the closest bleeding order).
Double check your booster vacuum line and connections. Power booster plunger rod is original to the car, and was originally inserted into the bottom hole on your brake pedal? All brakes adjusted with a slight drag? Zero leakage at the wheel cylinders?
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Cholesterol Bad Adrenaline Good Do it till your eyes bug out! |
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#24
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The 1" MC is correct for your application
I dont have a service manual handy right now but I believe that the MC/Pushrod clearance should be more like 1/64" .016". Tuffstuff aftermarket instructs no more than .020" clearance. Adding the RPVs will certainly help keep the cups extended but will you be able to notice a change in pedal travel? Only one way to find out. Dave
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68 GTO 68 LeMans 68 LeMans Convertible 79 Firebird Last edited by dmorg1; 01-21-2023 at 12:29 AM. |
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#25
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are there new wheel cylinders in there? I had a 67 with power drums that would pull to the right all the time under light braking, but was fine on a hard stop. no matter how I adjusted the brakes or how many times I bled the brakes, I couldn't get it to brake straight with a light pedal. I ended up just changing it over to power discs after screwing with it for a year since I'd planned on upgrading the stopping power anyways. a few years ago a PY member was looking for original wheel cylinders to rebuild so I pulled my stuff and took them apart to clean up the rust and caked brake dust. I found the left front wheel cylinder had some grime in the bore and the metal cups were hanging up, So a hard push on the pedal would get them to move, but they didn't want to move without the extra pressure. they weren't frozen, they just didn't like moving freely and weren't really dragging without the pedal. also not a drop of fluid leaking to indicate anything was up with it so i ignored them as a possible culprit.
Just a thought on something to check. you can pop the cylinders apart carefully and not damage them for reassembly just to check the bores in them. |
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#26
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Did you replace the 3 brake hoses?
I had a similar issue on an older car and found one of the front hoses had spilt the inner liner allowing fluid to pass into the outer part of the hose. It never leaked but you had to push a bit on the pedal before it built up enough pressure to activate the cylinders making them suddenly grab. If you haven’t put on new hoses, try feeling the old hoses for soft spots. |
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#27
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Quote:
I have also seen the metal swagged fittings rust internally and the swelling from the rust pinches the rubber hose causing a one way valve, where the brake spring pressure doesn't have enough force to return the fluid to the master cylinder, keeping the brakes applied partially. It causes premature wear of the brake linings, as well as excessive heat to be generated, similar to driving with your foot resting on the brake pedal while driving. Neither scenario shows anything abnormal from looking at the brake hose externally. Two instances I had forgotten about until I read the above post. |
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#28
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I wanted to come back to this thread to put a period at the end of the problem and thank everyone for their contributions.
I got my issue solved by finding a master cylinder that would accept the residual pressure valves. I was blessed to find a master cylinder on eBay that was not only unused, but it had the stamping correct for our firebird and also it had the bleeder screws on the master cylinder itself which aided greatly in bleeding the master cylinder during the brake bleeding process. That Master cylinder, although having provision for the residual pressure valves, did not actually have them. Finding a brake master cylinder rebuild kit for that 1-in bore with the two residual pressure valves, for the front drums and rear drums, was a challenge. But I was able to find one. All of that brought the pedal up to a good height. Then after installing all of that I still had extremely sensitive brakes. Like one-toe braking which was awful for a panic stop. I considered the advice that the 11 inch booster on our car was too much boost causing the grabby brakes ( even though, apparently for the power all drum brake 67 firebirds, the only option was an 11 inch booster) as was shared here in this thread. I obtained a nice Delco Moraine 9" unit and installed it and it's a world of difference. So the residual pressure valves gave me a higher pedal and the smaller booster gave me a good pedal that wasn't extremely sensitive. While going over all of this, I noticed that on the 62 to 67 Chevy Nova and Chevy 2, for the aftermarket power brake booster kits for their (all drum cars), those kits used a 9 inch booster. Thank you for all of your help and advice in this problem, it is solved. |
#29
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Glad to hear that you were able work it out. Good information, thanks for posting up your results.
Dave
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68 GTO 68 LeMans 68 LeMans Convertible 79 Firebird |
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