Race car brake question

Posted By: slantzilla

Race car brake question - 05/13/12 11:42 PM

I am going to use an NOS stock '88 disc/drum M/C on the Daytona. It has the factory discs on front and 10" drums on the rear. Will I need to reinstall the residual valves in the lines that were on the car when I got it? IIRC it has a 10# for the rear and 2# for the front.

I do not know what the M/C on the car when I got it was originally off of. It was junk anyway.

Thanks!

Dennis

Posted By: ProStDodge

Re: Race car brake question - 05/13/12 11:51 PM

Dennis,

I do know that if the master cylinder is located at a level BELOW the brake calipers/cylinders, the residual valves are needed to prevent the system from draining back. This is common with floor mounted master cylinders.

If you have the master cylinder mounted on the firewall in the "normal" position, I don't think they are needed.

Scott

Posted By: slantzilla

Re: Race car brake question - 05/13/12 11:57 PM

Thanks Scott. Master is in the normal place.

Posted By: Cab_Burge

Re: Race car brake question - 05/14/12 05:27 PM

I would leave the 10 lb. on the rear, nothing on the disc side

Posted By: ProStDodge

Re: Race car brake question - 05/14/12 08:22 PM

Quick google search and found all you ever wanted to know about brake systems.

The Brake Man

Posted By: Crizila

Re: Race car brake question - 05/14/12 09:31 PM

Quote:

Quick google search and found all you ever wanted to know about brake systems.

The Brake Man

Actually, that article is incorrect as to the main function of the residual valve. It's main function was to maintain a slight pressure ( 6 - 25 lbs ) in the line to the rear ( drum )brakes to prevent air from entering the hydraulic system past the lips of the wheel cylinder cups when the brake pedal is released and fluid returns from ther wheel cylinders to the master cylinder. It shouldn't be used maintain clearance between the shoes and the drums. Thats the self adjusters job - or in Dennis's case, his job. BTW, they ( residual valves )were mostly done away with when cup expanders came out in the mid seventy's. If your master is higher than your brakes, I would not use residual valves on ether the drum ( make sure your wheel cylinders have cup expanders - and they should )or disc side. If you do use one on the drum side, go with the 2 lb one. Brake drag of any kind is your enemy. Your return springs will probably over come the 10# one, but why take a chance. Since you are probably not using any self-adjuster hardware, you will have to adjust the rear shows periodically. No residual valve on the disc side. Are you using the porportioning valve that came with your brake system?

Posted By: slantzilla

Re: Race car brake question - 05/14/12 11:06 PM

John, there was no proportioning valve at all on the car when I got it. I was going to get an adjustable one from Chicago Connection when I start plumbing.

Posted By: Crizila

Re: Race car brake question - 05/15/12 12:17 AM

Dennis, my brake books don't go past 1980, but I would guess that your original system used a combination valve ( more than one function ) for a proportioning valve - and since you are going with the OEM front and rear brakes, I would suggest plumbing in an original porportioning valve, even though your cars weight bias has probably changed some. You can still install an adjustable porportioning valve in the rear line for additional brake bias adjustment if needed. Running the original P valve might not be too important if you are using a trans brake and if you DON'T experence any front tire skidding under light brake applications. Balancing the brake system cam become very "interesting" with a light car and major tire contact differences front to rear.

Posted By: MR_P_BODY

Re: Race car brake question - 05/15/12 01:42 AM

I wouldnt put the adjustable prop valve on the rear
line... its used to take the spike out if you hit the
pedal hard... skinny fronts would just skid if you hit
the pedal hard... the pressure will equalize after a
bit of time... but by then you not on a skike any
longer....the slicks have more stopping power over
skinny fronts

Posted By: Crizila

Re: Race car brake question - 05/15/12 02:45 AM

Quote:

I wouldnt put the adjustable prop valve on the rear
line... its used to take the spike out if you hit the
pedal hard... skinny fronts would just skid if you hit
the pedal hard... the pressure will equalize after a
bit of time... but by then you not on a skike any
longer....the slicks have more stopping power over
skinny fronts

Good point Mike has there. Maybe start with just the oem proportioning valve first? If you experience front tire skid, put the adjustable valve in the front line. My concerns for not using the oem P valve is that without it you eliminate the metering valve ( sometimes referred to as a hold off valve ). This valve prevents fluid pressure to the front disc brakes under light brake application. It holds off pressure to the discs until pressure to the rear brakes overcomes the rear shoe return springs. With a light car, skinny fronts, heavy rear shoe return springs ( or any combo of the above ) the front tires could skid under light brake applications - like during staging. If you decide to go with just the adjustable valve, Mikes suggestion of putting it in the front line might be the best solution.

Posted By: slantzilla

Re: Race car brake question - 05/16/12 12:29 AM

Thanks guys, you have overloaded my feeble little brain with information. Have to take time to difest it all now.