Overly Touchy / Sensitive Power Brakes

[68 P.O.S.]

As the title says, my brakes are extremely sensitive…to the point of it feeling like I’m going to be launched through the windshield. The initial travel of the pedal doesn’t do much, but once they start to catch, they work too well. I’ve already used the search function, but I haven’t come across a thread that seems to apply to my specific situation. So, the truck: 72 half ton with a stock 350 engine & trans and factory power brakes & steering. If ya’ll need more info, let me know. The story: bought the truck in 2014 and overhauled the brakes. For the front I replaced the rotors, pads, calipers, and hoses. For the rear I replaced the drums, shoes, wheel cylinders, hose and distro block, axle shaft seals, and fixed the e-brake. The brakes worked great when done. Fast forward a year and I did a suspension and front end rebuild to make it my daily driver because our van was stolen. I also wanted to install an Edelbrock EPS intake and a new brake booster w/master cylinder and combo valve from Frank’s Pickmups (great unit). Well, due to time constraints and me being tired and lazy from doing the suspension and front end work, I sent it to a local shop I heard good things about to install the booster and intake. Boy was I wrong. I finally get the truck back 2 weeks after it was supposed to be finished and the brakes are in their current condition. I was so mad I said screw ‘em, they lost my business and I tell everyone to avoid them. Anyway, I bought a new car for my daily and now I drive the truck from once a month up to once a week…depends on the wallet lol. So yes, my brakes have been like this for 2 years, but I’ve gotten used to driving it this way. I’m finally tired of it and want to fix it correctly. All the threads I have seen on this subject are for manual to power brake swaps and they all suggest moving the rod on the brake pedal to the lower hole. Problem is, I have stock power brakes and the rod is already in the lower hole. What are your thoughts and suggestions my dilemma? Any help I greatly appreciated

[harpo231]

Got a prop valve ?

[Aberdare]

Well... it sounds perhaps that as you said, the brakes work TOO well! You could start by checking how much brake pedal free play you have and adjust as required so that you have between 1/4 - 1/2 inch of pedal before actual braking begins -- IE, the master cylinder piston begins compressing. This free play is an essential part of a proper Master Cylinder / Booster installation.

Since you only mentioned the Booster is new and installed by someone other than yourself... this is where I would begin looking for a possible fault. The Proportioning Valve is Next as a possible fault location, as too much hydraulic pressure can also cause the problem you have described. **Thanks for mentioning it Harpo!**


Abe

[68is4me]

So you are saying the brakes worked good before, but you installed new master, new booster, and a new prop valve?

I would guess the problem has to be in the new parts somewhere. Maybe the master to booster rod is not adjusted correctly?

[harpo231]

[Bigdav160]

I was a little confused also. The rear drum brakes are suppose to apply before the front. This is the job of the metering valve. If it is malfunctioning,well, touchy brakes with front dive is the result.

[Jason Banks]

Post pictures of your booster, master cylinder and prop valve.

I have a similar issue with my truck. It is because someone put a 3/4 ton booster and master cylinder on my 1/2 ton truck. I have all drum brakes and the bore on the master cylinder for the 3/4 is larger and the booster is more powerful.

[RichardJ]

Start here. If I understood everything you posted, the booster was the last thing changed before it went sour.

[mike16]

I did that on a 66GTO one time. it ended up that my new master cylinder Piston Dia. was larger in diameter than the old one. pushed more fluid and with the booster too made it very sensative.