Technical Hard pedal, weak brakes

I have a 23 T bucket with '60s GM van 9.5 drums on it. I can't get strong brakes on it, it seems it's the fronts that are weakest. I replaced the master with one from Tuff Stuff that is very similar to a '60s era Vette m/c, a 2020NB, I believe. I used a Wilwood red residual valve for front and rear and installed them correctly. I also installed an adjustable proportioning valve on the rear line, which I haven't played with and have adjusted for full braking power to the rear. I've bled everything. The wheel cylinders, shoes, hardware seem fine. Any ideas? Thanks.

 

If you have a 1" bore master cylinder, that could be your problem. for a small T with manual brakes a 15/16" bore MC will work best.
The other thing to look at is your brake pedal length, the ratio of the pedal and the pivot to push the MC brake rod and activate the brakes. Should be around 7-1 I think.
Another thing is to make sure you have no residual valves built into the MC if you are using the inline residual valves.
I would check that out since it appears everything else is correct for drum brakes.

 

The master cylinder bore might be too small. The brake pedal leverage ratio might be too low.

 

Thanks for the help. I checked the brake pedal ratio - 9" above the pivot, 3" below. Not good.

 

Increased bore size reduces pressure, decreased bore size increases pressure. It is a ratio of MC bore diameter to wheel cylinder/caliper bore diameter.
From Wilwood Site: Common contributors to "hard pedal, won't stop" issues are an oversized master cylinder bore and/or inadequate pedal lever ratio.

 

I'm thinking of getting a Wilwood (or other brand) floor mounted pedal and a pair of single reservoir master cylinders. It won't look traditional, but I'd rather stop well. Besides, that racing style stuff looks good in it's own right.

 

The simple thing first, how old are the brakes hoses, they could be clasped inside, take them off and blow through them, the air should flow freely, (common problem)

 

Already tried that. I replaced the left front hose which did seem a bit restricted, but it didn't help. Everything else seemed ok. I think the 3:1 brake lever ratio might be a pretty big issue. The m/c bore is 1", but I have no idea what it should be.

 

1" is probably ok for the MC bore. Fix the pedal ratio first....been a while, but around 6:1 sounds more reasonable? I could measure one tomorrow...

 

With 9 in above and 3 below that is a 3 to 1 ratio - not good 7 to 1 is the deal Lucille!

 

You need a pressure range at the calipers of say around 900 to 1200 psi. (using Wilwood numbers). Line pressure is mechanical pressure divided by the surface area of the MC piston. If you use your leg pressure as 150 lbs. you can do a quick calculation to see if you are in the ball park. If your 3:1 mechanical leverage is correct then, 3x150 = 450 lbs into the MC. 450 lbs/1.0 in sq. = 450psi. Now does that seem like it could be an issue? Recommend mechanical ratios start at 6:1, at that ratio you would have 900 lbs into the MC and divided by 1 in sq you are still only around 900 psi line pressure (but closer to the useable range). With a 15/16 in MC piston and 6:1 pedal ratios you get 900 lbs /.9375 in sq = 960 psi (usable). With a 7:1 pedal ratio and a 15/16 MC you get 1050 lbs /.9375 in sq = 1120 psi. On my 65 Comet I had to go to a 7/8 MC, 1050 lbs /.875 in sq = 1200. So, you get the idea. At 7:1 mechanical leverage and 150 lbs leg pressure you have 1050 lbs into the MC, with a 1" piston that is 1050 psi, at 15/16" it is 1120 psi, at 7/8" it is 1200 psi, and at 1 1/16" it would be 988 psi (wrong direction!). Now the down side of all of this is with increased mechanical leverage or decreased MC piston size you increase the pedal travel distance, so it is a trade off, go to far and you have excessive pedal travel. There can also be a volume issue in some cases (the piston is not large enough to push enough volume of fluid).

 

What Fuzzy said. 7:1 ideal, 6:1 minimum. That would be the first thing I would change. Nothing will work properly until you do.

 

What they said. Increasing your pedal ratio (mechanical leverage) to at least a 6 to 1 ratio, 7 to 1 is even better. Increasing it to 6 to 1, you will be making about 770psi with 100lbs of leg effort, which is going to give you a pretty hard pedal.

With a 15/16" bore size master cylinder you will be putting out 1014psi with 100lbs of leg effort. The pedal with be a lot easier, you will have more pedal travel though.

If its 4 wheel drum, run two 10LB residual pressure valves (within 12" of the master if possible).

 

Pedal ratio as has been said. Too, how old are the brake shoes?? Cheap to get new, and if you didn't put them in there, it's time. The new material definitely makes a difference, even on stock systems.

Cosmo