1968 442, power disc/drum brakes, very hard pedal. Brakes work, but pedal feels almost like manual brakes, and requires significant leg effort. Pedal does not slowly sink when applied, just a hard stop. Replaced the master cylinder and booster, with not much improvement. I'm thinking it could be a restriction in the system somewhere. I hate to shotgun remedies, but I'm thinking of replacing the proportioning valve.... Anyone had experience with these going bad or plugging up? I'm going to replace the rubber hoses too, but I think if they were swelled inside from age, the brakes would hang up engaged and not release? Shoes & pads look fine, and rears have been adjusted. Seeking advice or similar experience solutions.

Thanks

 

out of curiosity what kind of booster did you install - aftermarket ones are fairly hit or miss and there are many stories of out of the box or short term failures with similar pedal pressure issues. what is your vacuum? And yes, the prop valve could possibly be sticking. If you didn't replace the booster and master as a matched pair, did you check to confirm you have the right bore piston in the MC?

 

We need more info. Are these original disc brakes or a conversion? If a conversion, are they factory or aftermarket? If aftermarket, what size rotor, what size caliper bore, what size M/C, what size booster? What is the engine, stock or modified? What is the idle vacuum level? Is this a new problem or has it always been the case with this system? And it is almost certainly NOT the proportioning valve - and FYI, the factory 1968 disc brake system didn't even use a prop valve.

 

I'm not sure if the front discs are original to the car or added by a previous owner. The calipers are single piston, not 4 piston as 68 original. Rotors are 11", new OPGI 9.5" Booster and M/C for disc/drum purchased as a unit. 400 Engine, re-built, but with a stock-like cam, nothing aggressive. Idles well, I can't tell you vacuum level right now, as she is tucked away for the winter. My 1968 Chassis Service Manual does indeed show a metering valve for 31-48 Series with Disc Brakes. I've attached a photo of my booster/master cylinder & plumbing. You are leaning toward a vacuum power assist issue versus a hydraulic pressure problem to the 4 corners?



I appreciate your help in diagnosing possibilities on this Joe.

Mike

 

So clearly not original to that car. They were added at some point, and the CPP master cylinder would suggest where they came from. Single piston is 1969-up.

OK, so that's a problem. The stock booster is 11". The only things that determine the amount of assist are the diaphram diameter in the booster and the vacuum level. A 9" booster has 2/3 the area of an 11" booster and thus only 2/3 the assist of the 11" booster. The next problem is the M/C. You don't know what the bore is. The larger the bore, the harder the pedal will be.

The metering valve is not a proportioning valve. All the metering valve does is delay pressure to the front brakes slightly to allow the rear shoes time to contact the drums before applying the front brakes. A proportioning valve reduces pressure to the rear brakes in proportion to pressure in the front (get it?) to prevent premature rear lockup. The 1968 full size cars used a separate proportioning valve in addition to the metering valve. The 1971-up cars with the combo valve incorporate both the metering valve and the proportioning valve in one housing. The 1967-70 A-body cars do not use a proportioning valve. The sizes of the caliper pistons and the rear wheel pistons were carefully selected to provide the correct front/rear brake balance.

A 9" booster will definitely require higher pedal force than the correct 11" booster. Depending on the bore of the M/C, that will also contribute to high pedal force.

 

All things not being equal (as per Joe's articulation of the variances in sizes) I would suggest you still determine your idle vacuum. If I recall correctly, you should have a nominal 17" Hg to 20" Hg. If you're below 17" Hg, you're going to struggle even further with a hard pedal. It's always best to know what vacuum you're pulling @ idle just in simple terms of the overall state of the engine's vacuum system & various components dependent on good vacuum.

 

Thanks all, I appreciate the info and guidance. Booster is indeed an 11", and I know the calipers are not correct for 68, but I'm going to focus on the vacuum being supplied. I did retard ignition timing at idle to provide a lopier sounding idle, and to cut down on some of the pre-ignition detonating when under load or accelerating, even when using 93 gas and octane booster. I imagine the retarded timing setting could be dropping the vacuum produced? Gives me stuff to think about while waiting for warmer weather....

Mike

 

I have been fighting a hard pedal from the day I bought my 71 cutlass 9 years ago. I have replaced the master twice, booster twice and now running hydroboost, combination valve has been replaced, new slave cylinders, shoes and drums in the rear and new calipers pads and rotors in the front. I have decided this winter is the time to get it right and am doing away with the combo valve because I think it is the root of all of my problems. Going to split the front and run an adjustable valve for the rear. If this doesnt fix it nothing will.

 

I wonder if Mike (mclark68442) resolved his issue?

 

Yep, that could do it. Back when I had to run my car through emissions testing, part of my "tuning for emissions" was to disconnect the vacuum advance and retard the initial timing. That would decrease the idle vacuum enough to make the brake pedal hard.

 

Any time that I've fought "hard brake pedal, poor stopping power" the FIRST thing I do is hit the highway and perform a HARD stop. Then get out of the car, and use an infra-red thermometer (or, in the old days, my hand) to see if all four corners are getting hot. The coolest brake(s) are likely the problem.

Plenty of times, I find the rear brakes aren't working (wheels cool) and the fronts are blazing hot. First suspect then is a plugged rear brake hose, but there are other causes to be investigated. I've replaced "too many" brake hoses that are plugged tighter than a bull's *** in fly season. Had a Toronado with both front hoses totally plugged. If the hose(s) are plugged, you tend to have a high, hard pedal. But that would be true with any hydraulic system blockage.

Both of my Luminas ('92 and '93) had defective power boosters that had "partial" assist. Very common in those years. Not totally dead, certainly not "right" either. The replacement boosters were significantly better with no other changes.