Line sizing on disc conversions?

[newschool72]

I am in the process of trying to free up my rear brakes for burn outs at the drag strip. Before someone says "wrong site" (LOL) I am just trying to get my timing set by looking for best MPH at the strip. My question is.....should the brake line size be the same front to rear if running identical calipers on all four corners? My 72 Camaro came from the factory with front disc/rear drums and uses 3/16 line on the front brakes and 1/4 line to the rear split . I am going to install a prop valve in the rear line so I can back them off in the burnout box, and at the same time have adjustability for bias on the street and auto cross. My brakes have had a fairly soft pedal sense I put it all together and I am thinking it may be because of the 1/4" line running to the rears. Another issue is finding the fittings to go from 9/16-18 out of the back of the master to 1/4" inverted flare. I can find the 9/16-18 to 3/16 inverted, so if it will help the pedal feel by changing the line size AND I can find the fittings for the conversion( and I did) I will change out the line to the rear as well.
So question is....will I get better pedal feel with the same size lines all around on my 4 wheel disc set up? I am running 4 piston calipers w 13" rotors on all four corners.

[astroracer]

Rather then messing with the prop valve EVERY time you do a burn-out and THEN resetting it before the run (remember, you still have to stop the car at the end of the track...)install a line-loc on the front brakes. Track officials may also frown on disabling the rear brakes if they see you doing it. A line-loc is MUCH safer and is designed for this purpose.
As far as the line size goes, I can't say. There are many things that can contribute to the dreaded soft pedal. It may be one of a few causes...
Mark

[lbluecamaro69]

I agree with the above.I think you can get a Hurst roll control kit for around 100 bucks

[newschool72]

I have come up with a way to adjust the prop valve from inside the car, similar to the way Nascar guys do it. I could just twist the knob to back it off in the burn box, then crank the pressure back in at the line. I want to do the prop valve because it could do double duty on the auto cross course. I could actually use it to dial in my brake bias ,instead of just having a line lock for drag strip duty.

[astroracer]

Do both! I would really hate to see you forget to reset that valve at the beginning of a run, because all it will take is once... Adjustability is good but turning it off is not. I hope you will reconsider your choice...
Mark

[newschool72]

I appreciate your concern , but the prop valve doesn't completely turn the rear brakes off. It only lowers it a max of 60%, which I hope will be enough to let me heat the tires up. I don't jam the brakes after I cross the line anyway. I like to ease her down at the end. No sense in chewing the brakes up for no reason, LOL.

[Red67Mustang]

Sounds to me like you need a line lock for safety, but also to save your rear brakes. All you can do with dialing up the prop. valve is make the rear brakes less effective, but there is still pressure on them when you're applying the front brakes to do the burnout.

With a line lock you apply the (normally biased) brakes then activate the lock, and then remove your foot from the brake pedal - fully releasing the rear brakes only. Way better for the rear brakes!

[astroracer]

Do the line loc and leave the prop valve alone... Don't think of it as MY concern but as YOURS and any PASSENGERS in your cars SAFETY! This should be front and foremost and burning up your rear brakes with the method you are proposing is not safe.
Mark

[bovey]

Agree with everyone above. LineLoc is the option - what you are planning to do is dangerous for you and fellow racers.

As for your question about line sizes your stock sized lines are find. Brakes are more about pressure and fluid moved - to move the calipers. Understand your entire system - if you make the lines bigger your current master cylinder may not move enough fluid to engage the calipers properly.

[Apogee]

Line size has nothing to do with anything other than maybe compliance, the amount of volumetric expansion due to pressure. For a given wall thickness, the larger the diameter of tubing, the greater the circumference, the greater the expansion under pressure. The master (MC) and the slave (caliper) relationship in a hydraulic system is fixed by the piston areas and line size makes no difference in the pressure or volume relationships.

Adjustable proportioning valves (APVs) are good for what they're designed for, fine tuning what should otherwise be well designed front and rear brake systems. As an optimization tool, they're great and I highly recommend them, they just aren't meant to be used for coarse adjustments. That said, your application is screaming for a roll-control/line-lock as that's exactly how they're intended to be used.

Otherwise, if you insist on plumbing an in cab APV for on the fly adjustments, I would recommend using either a detent style like most racers use (Wilwood #260-8420), or if you insist on the infinitely adjustable type, a good set of front and rear in-dash brake pressure gauges.

Tobin
KORE3