New Brakes, New Problem

[z-ya]

I did a bunch of searching, but didn't find a solution.

 

Here is my configuration:

 

Front: Wilwwod Forged SL calipers with 1.75" pistons (4). 12" x 1.25" rotors.

Rear: 240SX calipers and Z31 rotors.

 

Stock 72' 240Z 7/8" MC, Booster, and prop valve (worked fine all last year).

 

Here is the problem. When the engine is not running, the brake pedal is nice and firm. When I start the engine, the pedal gets a little bit softer. If I hold pressure on the pedal while the engine is running, then release the pedal, and then try to apply the brakes, the pedal goes to the floor, and I have no brakes! If I drive a bit, then apply the brakes, they work OK, but the pedal trave seems excessive, and it feels like the pedal is bottoming out before I'm getting full power to the calipers.

 

I have bled the brakes many times. I've also bled the MC many times. I know I will need to change the prop valve, but I would think that the brakes would function good enough for the street with the stock valve. I checked everywhere for leaks, and I haven't found any.

 

I had a feeling that I would have a lot more travel with discs in the rear, but this is way more than I expected.

 

Any and all comments appreciated.

 

Pete

[Tim240z]

Pete,

Have you tried adjusting the pushrod at the pedal (make it longer)?

[Pop N Wood]

I would replace the MC. When the pedal "occasionally" goes to the floor, then chances are the rubber seals are not gripping the cylinder walls tightly. Could be a slight bit of rust, some debris or maybe the seals are hardened from age.

 

I would also swap in a larger diameter MC. My 70 240 had that issue with the stock brakes. A larger diameter will give you a greater fluid volume. The Arizona Z Car site says to use one from a 1979-1981 ZX (http://www.arizonazcar.com/brake.html) I am not sure what the configuration is on a 72 vs. a 70, but when you do it make sure you route the front brake lines to the end of the MC with the larger reservoir. The ZX MC is swapped front to back relative to the 240 unit. Arizona Z Car use to have a write up on line, but looks like the info on how to swap the MC got dropped from their site.

[JMortensen]

The age old test for master cyl is to push soft and slow. If the pedal goes to the floor, the master is bad like Pop said. If you stab the brakes, the spike in pressure will make the seals expand and the master will work fine. So hard stab works, and soft push doesn't.

 

Pop is right, replace the master with an early ZX master. Get an adjustable prop valve while you are at it and try to dial in the bias. Rears should lock just after the fronts if you have it right.

 

Jon

[Afshin]

You definitely need to replace the master cylinder as already suggested. The ZX master cylinder is 15/16" diameter, which has sufficient volume to fill up the 1.75" pistons (4/caliper). The stock one is too small. I know that the 82-83 ZX turbo have 15/16" diameter, I don't know if it's the case for all the ZX master cylinders. Make sure it's the right size befor you buy it.

[z-ya]

Thanks for the replies.

 

I just ordered the 15/16s MC (79-81ZX). I was wondering if it would be worth replacing the booster with a 280Z unit? I figure this will give me abit more power assist. Anyone know if this requires fire wall hacking in a 72 240Z?

 

Thanks,

 

Pete

[Tim240z]

Pete,

I am using the Stock 240 booster with the 15/16th ZX master with Wilwoods up front and ZX calipers in the rear and the pedal feel is perfect.

Tim

[Guest Looking for apt in Alb Ny]

Hi Pete

I dont think the booster from a 280z will fit anything from 72 - earlier Z. They have different bolt patterns. Im sure someone has done it. My friend the other day found out the hard way.

Juan

[mrcheeze36]

I've heard that you can use the booster from the 260...is this true?

[z-ya]

I did a little more research BTW, and the 260-280Z boosters will fit with a slight re-drill to the firewall. I am sticking with the 240Z one for now. Since my original post, my MC appears to have fixed itself (maybe an air bubble somewhere?). I will be putting the inthe 15/16" one next time I open the system again thought.

 

Pete