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On my 68 327ci/350hp coupe the power brakes are a bit touchy. the power booster looks new, the master cylinder not so much. Saying the brake pedal acts like an off on switch is a bit of an overstatement, but when I tap the brakes the car reacts fast, very little travel. Over reacting and hitting the brakes hard can create wheel lock up. It has been this way since I purchased the car but got more concerning after replacing all the calipers and rotors. Could it have the wrong booster on it ? Is there an adjustment that might help. scared stiff, Atlanta
It sounds like you’ve got a the opposite problem of most people with C3s. Usually the brakes are too spongy.
Between the booster and master cylinder there should be a threaded adjustable rod. There’s a tool for measuring the gap, but you should have it at “a hair” maybe 1/16” of an inch.
Agreed, sounds like it's adjusted up a bit tight. Perhaps holding a small amount of pressure in the lines at all times. This is a bad thing. I'd definitely at least check it.
On my 68 327ci/350hp coupe the power brakes are a bit touchy. the power booster looks new, the master cylinder not so much. Saying the brake pedal acts like an off on switch is a bit of an overstatement, but when I tap the brakes the car reacts fast, very little travel. Over reacting and hitting the brakes hard can create wheel lock up. It has been this way since I purchased the car but got more concerning after replacing all the calipers and rotors. Could it have the wrong booster on it ? Is there an adjustment that might help. scared stiff, Atlanta
A picture might help.
Last edited by leigh1322; Nov 25, 2025 at 09:34 AM.
The manual brake masters require less pressure on the piston to provide equal fluid pressure -- perhaps you have a mismatched master/booster. Remove the bolts holding the master to the booster and gently pull it forward a few inches (keep lid secured on the master). Measure the bore diameter of the master: power brake masters should be 1.25" while manual brake masters are 1". The only year they were the same 1" bore with this dual reservoir master is '67, after that GM went to the two bore sizes.
But if he had the 1.0" master cylinder, with a booster, wouldn't that require more pedal movement than normal?
And the exact opposite problem that the OP is having?
I purchased the car from an estate of an older gentleman who have been doing a frame off restoration. Unfortunately, the car was put in the hands of someone else to finish putting it back together who is a real idiot. The older gentleman actually spent a bunch of money to make sure that he used the correct type of parts. I noticed that the fuel tank had that stamp/imprint on it that no one would normally see, but would've been the correct type. My hope is that the master cylinder was not adjusted properly when it was attached to the booster. I've already purchased the master cylinder. I was just hoping not to have to get into it until next summer. But the brakes are so skittish I guess I'm gonna have to jump in. I appreciate everybody's advice. And any other thoughts would be very much appreciated. Atlanta.
Is there any chance that I can disconnect the master cylinder from the booster without disconnecting all the lines and slip that tool in there to make the adjustmentsor do I need to just completely remove the master cylinder to get it done?
So I got the little tool, and just as Piersonpie said I was able to get the tool in there without removing the lines. Tight but doable. To my surprise there was a gap of 3/8 of an inch between the rod and piston. As near as I can figure, when I would hit the brakes the momentum of the booster rod as it hit the piston was what caused the off/on switch effect. Its still sensitive but I can now trust the brakes and not be afraid of them. Thanks to all for the advice and comments. I am going to wait until the summer to install the new master cylinder. Atlanta
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
It could also be the pedal rod is in the wrong hole...if you have a 4 speed the brake arm for the pedal has an upper and a lower hole...do believe the lower hole is for power brakes..research it
1969 Sensitive Power brakes
