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Low brake pedal/rolling stop


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1:46 AM
Feb 20, 2023
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Hello all. 68 coronet 500 was a power drum car that I am converting to a power disc front car drum rear. I have everything installed but I still can’t get this thing to stop or get a higher brake pedal. Right stiff disc brake kit using my pier drum 8 inch booster with a 73 duster proportioning valve that mount up to the factory lines from my power drum setup. New wheel cylinders new lines everything new but the booster. I have peer bled the system and did the old brake pedal pump up and hold method. I don’t see on air bubbles. Calipers bleeders are on top and rear brakes adjusted up. I am lost. Next I was told to adjust the booster to master cylinder rod but how far off could it be ? My pedal is just above the floor I can’t see a 1/8”of an inch making that big of a difference? I was told tge proportioning valve was a bolt in replacement for the distribution block. All the lines went right in thx for reading this. Help please also I only used the valve not the added brass port in the picture.


I don’t know if using a drum booster on disc would cause the issue you have. The pin travel may not be enough, or it may only reduce the assist. Checking/setting the pin depth would be a good first step. A lot of problems are solved by getting that adjustment right. That proportion valve looks like a PV6070MOP proportioning valve, if it is they usually do not come with this diagram or the bypass line and plug to use the metering valve. The plug and bypass line can be purchased at NAPA the line is 813-1154 and the plug is G60691-0004. The booster pin depth is important to get proper piston movement in the master cylinder. The link below describes that check, kind of a sales ad but good info. The Benefits of Booster Pin Depth Gauges (mpbrakes.com) Hopefully this helps, Good Luck

proportioning valve.jpg
I have not seen that diagram before. The one I have is the opposite on the master cylinder to the proportioning valve. I have those 2 opposite. I been emailing with cass at dr diff he said I would use the valve I have and not use the extra small brass block ? Also the diagram he sent me is the exact opposite let me see if I have a pic to share. Man am I confused. According to him the f reefers to the front brakes and the rear brake reservoir and the r is for the rear brakes and the front reservoir? Like I said I am confused thx

Sorry for the confusion on the diagram I posted. I posted it for the bypass line. The originator of that drawing reference to the rear is the master cylinder reservoir.
Your drawing is much clearer.
Booster pushrod adjustment is critical, a 1/8“ clearance x the pedal ratio equals a lot more than 1/8” extra travel at the pedal. Rear brakes out of adjustment will also result in a low pedal.
Did the MC get bled? Can bleed system repeatedly, but air in the MC stays there. As posted, rear brakes need to be adjusted so linings are just giving a slight drag on the drums, this can be a cause of a low pedal. Problem I had once was a bad booster. New check valve? Can test it to make sure it’s working. Can also check the booster using a pump vacuum gauge to make sure it holds pressure. Motor pretty ‘stock’? If not, built up motors having a bigger cam doesn’t generate the minimum vac needed to maintain ample inches. Installed a reserve can on one of my builds. Another thing to check with a partner with wheels off, is the action of the calipers press and release. Had an issue once with the pads binding, not fully retracting causing a low pedal.

Lol, there’s more…but a few thing just in case haven’t been ruled out..