brown7373

My brakes seem fine unless I really have to apply them hard. I don't think I can lock them up no matter how hard I apply them.

My car was originally a power drum car that I converted to front disc using all parts from a donor Cutlass, new pads, rebuilt calipers and hardware, bearings, hoses correct combination valve and junction block, new master cylinder for disc, remanufactured power booster for disc (Steve Gregori) etc. And here is the strange part. The power drums were the same...I couldn't lock them up either. I have bled them several times, but the pedal is normal and does not EVER go to the floor. During normal driving, the brakes are great and you would never suspect any problem. But if you have a need for a more sudden stop or slow down, something is wrong.

Again, the issue was the same with the original 4 wheel power drums and also with the conversion to power disc/drum.

Does the rod need adjusted? Any ideas?

Rocketbrian

I've always wondered this about my 71 Cutlass and 442. With the stock system and the best replacement parts I could find, there is no way I can lock up the wheels. I have always felt the proportion of force on the brake pedal doesn't correspond to the clamping force of the pads on the rotors. A deadness feel is how I would describe it.
Perhaps they aren't designed to lock or the brakes are undersized for the weight of the car.
I'm almost relieved to hear of someone else comment on this. Other opinions will be interesting.

Fun71

No, I don't think so. Both my old 1970 Cutlass power disc/drum and my current 1971 convertible easily lock the brakes by standing on the pedal.

One thing I have experienced in the past is the front brakes don't grab as hard after the car has sat for some time. A bit of corrosion builds up on the surfaces of the caliper piston and causes "drag" on the seal as the piston tries to move outward. Some years back it got so bad on one car that I had to pull the calipers and move the pistons back and forth in the bores until they moved freely. The brakes worked much, much better after that.

brown7373

I drive my car regularly, so it isn't from sitting. I have had a 69 GTO and a 69 Grand Prix which have the same brakes and they could lock them up if the brakes were hit hard enough.

Gary M

stock or big cam? Big cams need a vacuum pump for the brakes to work better.
brake lines in good shape?
check the line from manifold to booster

Fun71

Also check to make sure the rear drums are adjusted properly. That can make a BIG difference in braking ability. My Jeep Wrangler has crappy rear adjusters and the rear drums regularly get too much slack the braking literally sucks. Adjusting the drums makes a HUGE improvement in braking.

joe_padavano

First of all, locking the brakes isn't necessarily a good thing. There's a reason why new cars have ANTI-lock brakes.

More to the point, the ability to lock the brakes depends on a lot of things, including the friction between the tire and the road (higher friction makes it less likely that the brakes will lock), friction between the pads and rotors/drums, glazing on the pads, wear in the M/C, leakage in the booster, restriction in the rubber brake hoses, and correct pedal ratio.

If your conversion didn't use a brand new master cylinder, I'd start there. The piston seals in the M/C wear and can leak internally. You won't see any puddles, but brake pressure will bleed off internally to the M/C.

brown7373

Stock cam, with plenty of vacuum (I think) but I will check that.

Joe, I had the same issue when it was power drum brakes, and was one of the reasons I converted to front disc. And radiusing (spelling?) the shoes didn't fix the problem. It isn't that I want to lock the wheels, but when more stopping power is needed, like when someone pulls out right in front, I find I am standing on the brakes and they aren't providing enough stopping power. If I was doing a zero to 60, and 60 to zero test, I would lose big time because the car doesn't have enough reserve stopping power. I am not comparing this 45 year old car to a new model with anti lock brakes, but to ANY older car I have ever owned or driven. It kind of feels like you reach a point where the pedal can not apply any more power to clamp the pads, or rear shoes. But pedal feels normal and does not go to the floor, doesn't pulse. Also, the car always stops straight as an arrow.

joe_padavano

Did you use the same brake booster when you converted?

brown7373

No, I got a rebuilt original from Steve Gregori for a disc/drum car.

brown7373

I will connect a vacuum gauge in the line to the booster to see how much vacuum I have at various braking conditions.

joe_padavano

Well, what HASN'T been replaced in the system? Short of putting a brake pressure gauge in the system, you may want to look at whatever hasn't bee replaced. Rubber hose to the rear axle? Rusty hard lines that might be expanding under pressure? Weak rear wheel cylinders? And finally, are you SURE the system is fully bled? Do you have a combo valve or the separate metering valve/distribution block? New or used?

brown7373

All 3 hoses were replaced. New wheel cylinders. New steel lines from master to new combo valve and distribution block. New rear lines. Main line (front to back) was not replaced.

I am going to re-bleed, but the brakes have been bled several times, both the disc/drum and the 4 wheel drum.

If the push rod is misadjusted too short, wouldn't it prevent the brakes from full travel? Is there an adjustment procedure?

Mrgadget

My '70 had a brake issue similar to yours.......disc up front and drum in rear, couldn't lock them up.
My mechanic friend put it on a lift and there is a brake bias valve under the driver seat area, it gave a direction to go for less rear bias and more front...........it now will lock them up.

brown7373

Are you referring to the combination valve or a proportioning valve? Can they be adjust the bias, or is yours an aftermarket?

joe_padavano

The combo valve COMBINES the prop valve and the distro block (hence the term). The factory style external metering valve IS the prop valve for the 67-70 cars and is only used with a distro block.

brown7373

I put a vacuum gauge into my booster line. I have 16-18 inches of vacuum when off the throttle and applying the brakes. There is a slight drop to about 15 inches but nothing out of the ordinary when braking hard.

I have always heard to NOT change the length of the push rod. I have not changed it, but my brakes have been like this since I bought the car. So I don't know if a previous owner misadjusted it. I don't recall seeing any procedure to check or adjust the length, but I will look at the service manual again. Any thoughts on the correct length or a checking procedure?

Mrgadget

[QUOTE=brown7373;1017575]Are you referring to the combination valve or a proportioning valve? Can they be adjust the bias, or is yours an aftermarket?

Mine is an after market proportioning valve from Summit. It was on the car from PO.

https://www.summitracing.com/parts/c...ic-performance

Rocketbrian

If you look in the service manual they speak of a gauge, using the terminology of Go or No Go, rather odd to the uniformed. There doesn't appear to be any method for adjusting the rod. It seems aftermarket boosters may offer this feature, as the factory rod just pushes in to a stop. So, I don't know if extending the rod would offer any improvement. I am suspect sometimes of those brass combination valves, vs. the factory cast iron units, perhaps set up different internally for front to rear bias. Just a thought.
Any updates? I hit the wall on this issue with my cars.

oldcutlass

I have a buddy that is having a similar issue with his 66 Chevelle SS. As soon as we get a few projects out of the way we are going to tackle his. He has a front disc brake setup from speedway and an after market 7" booster.

brown7373

But as I posted earlier, I had the same issue with the original factory power drum brakes...and that was even after I had the shoes radiused so they fit the drum correctly. I then used all disc parts from another donor Cutlass, including a correct combination valve and a correct booster for disc from Steve Gregori . BOTH the power drums and power discs feel like they only are working at about 70%. Fine for normal driving but not if you need a more aggressive or panic stop.

Mrgadget

I had the basic same setup as you and the same brake response, not good stopping power, then we saw the brake bias valve, adjusted it for less rear and more front, it made a world of difference.