Weak brakes on new disc/disc setup

[switchblade327]

My car has been in a resto shop for the past couple of years while I've been abroad for work but I'm back now and trying to take care of some of major issues that are still plaguing it, to get it back on the road.

Anyway, my '55 Bel Air (which I know is not a typical PT car but I'm trying to build it to perform) has been switched to 4 wheel discs but when test driven, it barely stopped, with apparently not enough pressure. I wasn't the driver and the car isn't running now so I can't describe it better than that. They've bled the crap out of the lines and still nothing. There are no apparent leaks so we suspect a mismatch of the components.

The front brakes are a kit from Danchuk, with 13" corvette rotors and PBR calipers like this:
http://www.danchuk.com/ItemForm.aspx?item=12817 I went with it because it was a big disc, good price and didn't modify track width like many kits for these cars do.

The shop owner was concerned that the calipers of this kit only grab about 50% of outer edge of the rotor and not the entire surface of the disc. But Danchuk has been selling this kit for years with no complaints about people not being able to stop. It may not be the best brake kit but I can't believe they'd be selling an inherently dangerous design.

The rears are 12.25" Wilwoods and were done much later when the stock rear axle was swapped to a 9" (hence the total mismatch).

Before I continue, am I crazy to run this combination? Of course matching brakes would be nicer but is this combination at least safe and usable?

The master cylinder is a Corvette disc/disc (not sure what diameter) with booster. The proportioning valve is a nonadjustable GM one, possibly disc/drum.

My thought is that the master cylinder is the most likely culprit. The proportioning valve is probably wrong for this setup and will be changed if incorrect but if I understand (nonadjustable) proportioning valves correctly, neither one would deprive the front brakes of adequate pressure to stop.

With having discs this big, is a stock style disc/disc master cylinder going to cut it or do I need to go to a smaller diameter aftermarket master cylinder?

Are the brake lines a potential problem here?

Any recommendations for diagnosing? I have about one month to get as much done as I can on this car before leaving again so I want to attack this problem with a vengeance.

Any help in troubleshooting this would be very much appreciated.

[Apogee]

The front brake kit you have is basically the same as a 1988-1996 C4 Corvette setup with the 13" HD rotors. This is what Baer has used for their "Track" kits for decades. The front calipers have twin-38mm pistons (A = 3.53 sq in) and as such will require a relatively small bore master cylinder in order to work properly. How small should be determined by the booster you're running (which you didn't specify) and the vacuum levels being supplied to it, however I would think a 15/16" bore unit is where you'll want to be in the end. Based on the peformance you've described, you're probably running the 1-1/8" bore C3 power disc/disc master cylinder now.

The Wilwood kit in the rear shouldn't be a problem, especially if you remove the OE disc/drum combination valve in favor of an adjustable proportioning valve. Most rear Wilwood kits using the 12.19" rotors and 4-piston Dynalite calipers have piston areas equal to 3.00 square inches. For comparison, the OE C4 Corvette piston area was ~2.47 sq in.

As long as the brake lines and hoses are not collapsed and restricting flow, they should not make any difference with respect to the caliper pressures as volume has no impact on pressure when treating brake fluid as an incompressible fluid, which it may as well be for the purpose of this conversation.

First step is to determine your MC bore. Next, post up the booster size and type that you're running, e.g. dual-7", dual-8", dual-9", single-9", single-10.75", etc.

The only thing that hasn't been brought up yet is pad compound, however that's something you can look at after fixing the major issues with your setup.

Tobin
KORE3

[Hydratech®]

Imagine being able to completely and instantly upgrade your braking to a much higher level of response without even having to touch brake fluid or unbolt the wheels:

http://www.hydratechbraking.com/Chev..._Fullsize.html

The brakes you have at the wheels will dramatically rise and shine once hit with higher power levels.

We are also now open to the public:

http://hydratechbraking.com/contact.html

[GregWeld]

Switchblade --

The other possible thing to look at is your pedal ratio on the brake. We don't know what the car came with in '55... manual or power... and the pedal ratio is important as it's a multiplier of the power you apply with your foot.

It should be 6:1 for manual and 4:1 for power brakes.

Also --- As Tobin pointed out -- the Master is CRITICAL.... larger bores make LESS pressure -- smaller bores make MORE pressure.

There are diagnostic tools for pressure measurement at the calipers. Don't guess - invest in these relatively cheap pressure measuring tools - or find a competent shop that can take these measurements.

[Rick Dorion]

With Tobin's input I was able to get acceptable braking with my C4 setup in a manual mode but I had to drop down to a 13/16" bore master cylinder.

[conekiller13]

Apogee has it right with M/C bore vs Caliper bore ratios. If that isn't right You will never get a good pedal out of it.