Manual Brake System Trouble Shooting: Expected Line Pressure?

[BANKO]

Hey guys, I converted from a power booster w/ front disk and rear drum to manual brakes running Z06 6-piston fronts and still have the rear drums (for now).

Under panic braking I can't lock up the fronts and I feel a little scared at how much pedal input is required to slow down. Not sure if I have air in my MCs or if my proportioning valve is reducing the pressure.

Here is the setup I have:

Wilwood 6.25:1 pedals
Wilwood High Volume 3/4" bore (front)
Wilwood High Volume 7/8" bore (rear)
Wilwood adjustable proportioning valve

My balance bar is set equal front to rear and the proportioning valve is full open, with 57% reduction to the incoming line pressure. I think the pedal is too hard for being a 3/4" bore and have had others drivers give me the same feedback. Next action I'm going to take is to check the line pressure at the front calipers, does anyone know how much pressure is expected?

[Matt@BOS]

I can't tell you exactly what line pressure is expected, and don't want to throw out a number that is wrong, but I might be able to help with the pedal setup.

I'm running dual master cylinders for the brakes, and I have Wilwood W6/W4 calipers. The W6s, (if I recall correctly) have a similar piston bore volume to the C6 Z06 calipers, but I don't know the rear Wilwood calipers would compare to the drum brakes.

I am using 3/4" front and 7/8" rear master cylinders and I have the balance bar set significantly towards the front brakes, and no proportioning valve. From my experience the 7/8" seems to move enough volume in relation to pedal travel that you need to make sure the pedal is pushing a lot more towards the 3/4" master cylinder. At the last RTTC I had the bar centered, or maybe even towards the rear after some tinkering the previous week, and I could not get the car to slow down at all!

As for the pedal feel. The pedal in my car is pretty firm. It isn't what I'd call rock hard, but it doesn't feel anything like manual brakes with a single tandem more master cylinder. There isn't a ton of pedal travel, and the pedal isn't what I would describe as "springy" like something you'd find in a new car.

If you have any other questions, or a better or more specific description I would be happy to try and help further.

[transam]

Wllwoods site has some great tech advice for this topic as well. I have manual brakes with a willwood master and i had horrible pedal feel, u needed both feet to stop the car and i couldnt lock the wheels, i looked on wilwoods site and i was reading on the ratio you need for your brake pedal ,mine was way off,i redrilled and relocated the push rod and i got a really good pedal feel and stopping power. worth looking into.

[Payton King]

correctly size the masters to the calipers. May be hard in your case with drum rear brakes. Personally I would not run the proportioning valve.

Most calipers do not want more than 1200 psi. You can get some caliper pressure gauges from Longacre for about $100 for 2 gauges and a cool metal case. For C5 sized brakes front and rear I was getting 1100 psi front and 850-900 rear with about 100 lbs leg pressure. I ended up with a 3/4 front and 3/4 rear on mine, but the rear sizing was due to 335/19 tire that was 27 inches tall. It could take more rear brake and still be ballanced.

I am going to guess that your problem lies in what brake pad you have up front. If you are running a street pad, I would change to something more agressive. I tried the Wilwood BP10's and could not get my car to stop no matter what I tried. Went to a Carbotech Ax6 (autocross pad with good cold bite) and it was a night and day difference. They will be dusty and will make some noise. Mine never made any noise, but I have heard other people say they do.

I have not tried but heard good things about the Hawk HP+ as well.

[Blake Foster]

Originally Posted by Payton King

correctly size the masters to the calipers. May be hard in your case with drum rear brakes. Personally I would not run the proportioning valve.

Most calipers do not want more than 1200 psi. You can get some caliper pressure gauges from Longacre for about $100 for 2 gauges and a cool metal case. For C5 sized brakes front and rear I was getting 1100 psi front and 850-900 rear with about 100 lbs leg pressure. I ended up with a 3/4 front and 3/4 rear on mine, but the rear sizing was due to 335/19 tire that was 27 inches tall. It could take more rear brake and still be ballanced.

I am going to guess that your problem lies in what brake pad you have up front. If you are running a street pad, I would change to something more agressive. I tried the Wilwood BP10's and could not get my car to stop no matter what I tried. Went to a Carbotech Ax6 (autocross pad with good cold bite) and it was a night and day difference. They will be dusty and will make some noise. Mine never made any noise, but I have heard other people say they do.

I have not tried but heard good things about the Hawk HP+ as well.

I was also going to say pads, but our red 69 has 14" proplus front and rear with a 1" wildwood master and it is awesome with the pads supplied by Baer. my nova has 3/4 f and 7/8 rear with 14" proplus and it to is good but I do run a wildwood auto x pad

[Matt@BOS]

I should also mention I've run BP20 (street) E (autocross, okay cold. needs a tiny bit of heat to work best), and B pads (road course, terrible cold) and could lock all of them up during a panic stop. The B pads took a ton of effort though.

It doesn't take much to dial out too much front brake on the balance bar. With rear drums I bet it would be down right scary trying to stop if out of adjustment.