'66 Power Drums, Hard pedal, weak braking won't lock wheels???

Hi all,
Recut all drums, new wheel cylinders, new shoes, 67 drum/drum master (dual resv), replaced rubber lines, good metal lines all around, good check valve/grommet,hose. I am wondering if the booster is bad or a vacuum can is needed. Mild 425 cam in a fresh 445 that runs nice.

But the pedal feels hard with limited travel, and when I hit the brakes hard it just does not feel like positive braking. If I stand on the brake pedal and give a little gas the car will start moving. What am I missing here? What would you guys suggest can be checked or tested?

I am concerned if there was an emergency situation the car would not stop fast enough.

 

Sounds like the booster is bad. How much vacuum does the engine make?
To check the booster :
With engine off, pump brake pedal a few times to bleed off any stored vacuum in the booster.
Then, with brake pedal depressed, start engine. The pedal should sink about a inch. If it doesn't the booster is not working.

 

When installing "New" brake shoes & re-surfacing drums you need to give the "New" shoes time to "Wear" into the new O.D. of the drums. We used to have a tool that would mount to the hub/spindle & actually cut the "New" shoes to the I.D. of the re-surfaced drum. The pedal will be "Soft" & not too responsive. I usually do the 30-30-30 rule. 30 stops @30MPH with a 30 second cool-down in between. Just a "Little" harder than normal. May want to wait more than 30 seconds as it takes longer for the drums to cool than discs.
Just my thoughts.


Tom T.

 

Agreed, sounds like a bad booster. With all new components, your head should damn near hit the dash during a panic stop.

 

Thanx Guys,
I will try to get it out for testing later this week. The weather went bad again so she is in the garage for a few days. Kind of glad it is going to rain some to wash the salt off our roads.
UntamedKitty

Oh Yeh, found this little kitty at the Milwaukee Pabst Blue Ribbon Rod Run last summer! It was a hoot! Look for it this summer too!
Yes, that would be a PBR at 9am on Sunday...Breakfast of Champions!

 

Did the pedal pressure test over the weekend. When I started the car up, the brake pedal moved up about 3/8 of an inch at most.
Could this limited movement also be from the MC to pedal rod being out of adjustment?

 

Hey brother I JUST went through some similar issue with my brakes. Although my issues were not as severe, I have a feeling you might be in a little of a similar boat.

The hard pedal with limited travel sounds like you are not getting enough assist. There are a few things that could cause this:

- Wrong pedal ratio. If you have a modified (home engineered) brake pedal OR the car has swaped between boosted and non-boosted brakes, then you might not have the correct pedal ratio. Power brakes should have a ratio of 4:1 to 5:1. If it is too low, your foot is does not apply enough mechanical leverage to the booster. To check yours, take a look at this website:
http://www.mpbrakes.com/techtalk/how-to-series/correctly-calculating-pedal-ratio

- Low vacuum. Boosters need a good amount of vacuum to work properly. There are varying opinions on how much you need, and different boosters need different amounts. To be on the safe side, 15"+ seems to be where you want to be. Anything less might not give you what you want in terms of brake assist, and if it is sufficiently low the booster might not work at all. You really need to measure your manifold vacuum before you do anything else. If it is low, you will need to either fix the low vacuum condition (caused by leaks, worn engine, etc) or if it is due to your cam, then you might want to look into a hydroboost or electric vacuum pump. A vacuum canister is not going to do you any good. It will store vacuum, but it cannot create any more vacuum than your engine can. So, if you only pull 10" of vacuum, you will have 10" of vacuum in your canister. It is simply a stored reserve, which if you lose engine power, your properly working booster should be rated to make a full boosted stop due to the stored vacuum it has internally.

- Bad booster. Your booster could have a vacuum leak, bad check valve, or is just old and worn and just doesn't work right. There are a lot of tests you can do such as the test mention above, but that is not the only one. Mine passed that test, but I could not pull more than 12-15" of vacuum on it with a mighty vac, so something was up internally. Try pulling a vacuum on it. I think if you can hit 20" and hold that for 5 minutes without a lot of leak down you are probably good. When I removed mine, the input rod had a LOT of play compared to the new one. Also, a leaking master cylinder can dump fluid into the booster and cause a ruckus inside as well.

- Wrong master cylinder: If you changed this part out you might have bought the mistakenly received the wrong one. Master cylinder bore size is very important. A larger bore MC will give you less pedal travel with a hard pedal. A smaller bore MC requires more travel, but can generate more braking force with a lighter pedal force. Think about this: if you apply 100 lbs of force to a pedal, that force is amplified by the booster and applied to the MC bore. Remember that pressure is force/area. A smaller diameter bore MC puts more pressure in the lines than a larger bore since the force is applied over the smaller cross-sectional area of the MC bore. However, to move the same volume of fluid, a smaller bore MC must move farther than a larger one, which gives you the longer pedal travel. The thing is, every brake system needs to be set up with the right MC bore size to match the rest of the system. The wrong bore size can cause weak brakes or incredibly touchy brakes that lock up too easy. I am not sure what you car needs, but if you know what size bore you have we can go from there to try to figure out if you need anything different.

- Bad plumbing: I am not sure what kind of valves the older Buicks had on them, but some cars have a valve that has a safety shuttle inside that will block off either the front or rear system if there is a large pressure drop in that system. Again, not sure if your car has one, but if it has tripped, maybe you are only operating on the rears?

Another way to objectively test brake pressure is to get a brake line pressure gauge. They are only like 30 bucks. You screw it into a bleeder port and have someone press the brake pedal in a normal fashion with the engine one. I know disc systems like about 1000 psi. Not sure about drum, so you would have to look it up.

Hope this helps!

 

"back in the day" you could buy oversize linings, or get the new lining "arc ground" to match the worn drum

 

Great info guys, I will be chasing this one soon!
It stops OK with some extra room for safety, but in an emergency, I would be worried.