TheSkunkWorks

Having already been sufficiently spoiled by "tools" such as adjustable brake bias, this is something else I intend to address on my C3. However, I've yet to select specific kit myself. What I can offer is that I would definitely stick with a dial type over any levers, and I do like the idea of the Heidts' built-in sensor, tho I'd prefer something in the way of cockpit adjustability (again, spoiled). Agree the dual master would afford the most latitude, while being a more difficult mod.


As for the whys and wherefores of Paul running into this problem in the first place, there's no doubt the factory played it safe by designing front bias into the standard system. (Yes, J56 has included an adjustable prop valve) So, I'm a bit surprised more hard-charging CF'ers haven't squawked about this already.

redvetracr

whats to squawk about, a factory type MC, J-56 calipers and a LEVER TYPE Wilwood adjustable proportioning valve (adjusted one click, red lever in the pic below) have worked great for me, 36 vintage races, six overall wins in the big car class at Road America with a fast lap of 2:25.0 (which is flying) and the only time I ever lost my brakes was my very last race where I had two rear pads swapped side to side....my last race, my last win.

69427

IIRC, some of the IMSA Corvette guys ran some front calipers in the rear. The Greenwood widebody type cars had a crapload of tire in the rear, along with a good size spoiler and some weight redistributed rearward.

Most anything is possible if someone is willing to do a bit of fabrication.

TheSkunkWorks

By no means would I suggest arguing with the success you've had with your setup. My "squawk" comment was meant to be in reference to the bias provided by the standard, non-adjustable factory system.

The only time I've ever "lost" the brakes (and on a street course, no less), I put the failure down to my having not done a better job managing bias during the race. Lesson learnt, I got myself very acquainted with tuning bias, and just happened to have found the ****'s virtual infinite fineness particularly well suited to changing conditions such as damp to drying. JMHO, YMMV

69427

I'm not following what you mean there.

ignatz

I could maybe see how to mount rears onto the fronts by cutting off their tabs and drilling new mounting holes






but going the other way escapes me. It might help that the fronts have a threaded through mounting hole.

I will look at it but I am pretty much the world's worst machinist. No luck on a search for some kind of adapter either

________
maybe something like this?

69427

Friend of mine raced in IMSA in the 70's. I'll have to give him a call and ask how it was done.

Just off the top of my head, I wonder if just cutting and re-welding one ear on the rear caliper bracket (that bolts to the bearing carrier and trailing arm) would do the trick. (In a somewhat similar situation years ago I had to fabricate a custom rear bracket when I put Wilwoods on the C4 suspension under my '69.)

racervette69

Used a brake box with twin tilton master cylinders. Smaller bore for the front and larger for the rear. Manual balance bar is adjustable from inside the car. Rarely adjusted it one it was set. Adjusted the bias with the car on jacks. Knew exactly the feel and balance I preferred.

ignatz

Racervette69: A little extreme for me. I am sticking with my caliper replacement plan and will post separately if I go through with it. Your cup holders are nicely placed however!

racervette69

Lol, easy to see the brake fluid in the cup holders. Agree, this was added during its GT1/IMSA days. Changed to different rear cylinder bore size on my old EP 002 racer. It worked well. Changed to tii cylinders.

racervette69

terrys6t8roadster

I'm running 305/680 r18 out back with 285/645 r18 up front. Had similar braking problem [Wilwood brakes and master]. Cured the problem with an inline adjustable proportioning valve from Jegs [about $35] in the rear brake line and removed the OEM valve. Mounted it up close to the M/C for ease of adjustment. Seems like once you get it set you don't have to mess with it. The only thing I didn't like about it was it was set up for 3/16 line and had very small porting. So it was off to the machine shop for rework to increase flow and port size to our 1/4" line. T

ignatz

Me too!

Terry: I am unclear what "similar braking problem" you experienced if you put a proportioning valve on the rears. The OP's complaint and mine are that the rears don't seem to be doing much work with the stock or Wilwood setup.

Grateful for the pix of your valve installation. I bought a Tilton valve and am looking it over and, as I said, thinking about installing front calipers on the rear to get more adjustable rear authority.

As to the 3/16" port, that bothers me a little but it just limits flow not pressure. I plan to take a look at MC capacity and volume change before (and if) I dive into this modification.

pauldana

As I stated earlier, the 79's have a proportioning valve stock... not sure what year they started, but it did go through the end of the C3's in 82,,,, so, What i have been trying to find out is what % our stock valve proportions to? The adjustable unit is made to do the backs, but should go all the way down to 0, and that should work,,,, i.e., if the stock is set say, at 20%, if i can dial down to say 10%, that would be quite a gain in the rears...

pauldana

thats what I'm thinking of going with... But I think there is an adjustable unit with our stock size and thread pitch.

ignatz

I don't know about 79's but can tell you because I've measured the pressure in the fronts and rears simultaneously, that the brass distribution block in my 69 does nothing to reduce pressure in the rears. It's effectively identical and the braking effort is controlled by the piston area.

You can find out for yourself what you have with this

http://www.summitracing.com/parts/lng-44145

Before you commit to dialing down the rears with another valve suggest you actually measure what you have.

As to a proportioning valve with our stock size and pitch, please post the details. I have not found such a device.

pauldana

did you put the adjustment side on the front or rear brakes?

pauldana

http://www.ebay.com/itm/HOT-ROD-PROPORTIONING-VALVE-ADJUSTABLE-LINES-/261770083944?hash=item3cf2b67e68&vxp=mtr

This auction is for a new CPP Adjustable proportioning valve block. This is what CPP is all about, for years we have seen people sell the proportioning valves and had you mount them anywhere you wanted, CPP is the first company to ever make a kit with the valve, bracket and lines ready to bolt to your Master Cylinder. The lines are for the traditional GM master cylinder with 9/16-18 and 1/2-20 making this a plug and play job at your fingertips! So many cool features to the block read below this is a must have item!
(Part# PVKS-APVDB)

This Valve is an absolute must for just about any hotrod or streetrod application. Understanding the valve is pretty simple. The front to rear brake balance is partly controlled by the proportioning valve. The proportioning valve is used to reduce the rear brake pressure that exits the valve. The pressure required to the valve to start reducing the pressure is known as the split or knee point. After the incoming pressure is higher than the split point, the pressure leaving the valve will be less than the pressure entering the valve. Lets look at an example where the brakes are applied in a hard stop. (For this example assume the valve is adjusted to a split point of 500psi.)The front and rear pressure will rise equally until the split point has been reached(500psi.) After that the rear pressure will rise at about half the rate the front does. With 1000psi at the front brakes there will be about 750psi to the rear brakes.(The first 500psi is equal; increasing the front another 500psi will increase the rear by 250psi) The proportioning valve uses standard 3/16" tubing flare fittings at all tube ports, and a 1/8 pipe connection at the brake light switch. This valve is a stunning show black and comes with the wiring pigtail and all the flare nuts for your brake lines and very detailed instructions making the installation and the understanding of the valve a snap! (This is not a direct factory replacement for original blocks and some modifications might be necessary)

This is a brand new valve put out by CPP and is a very popular valve. This is a Great opportunity to save some $ on our top quality Adjusatble Prop Valve.

racervette69

The old and outdated way of setting or checking your bias. With the car on stands. Have someone at the front wheel rotating it. Have someone in the car. Apply pressure on the brake pedal until you can no longer rotate the front wheel. Have them hold that pressure. Takes someone with good pedal feel. Then go to the back wheel. You should be able to rotate it,
but should require considerable amount of pressure. We would repeat a number of times.

Never tried this with any type of assisted brakes. Pedal feel is the reason assisted brakes are rarely used on a race car. Direct feed back from the brakes.

Anyone remember the Safe T brake, thirty plus years ago ? Worked like a shock in the brake line. Advertised to stop brake lock up on an unloaded tire. Attempt of anti lock on race cars. Not sure how well it worked. Most series banned it.

pauldana

OK.... Just got off the phone with Justin over at Willwood Brakes,,,,
Here's the scoop... YES, it is a proportioning valve... and

wait for it.... its'''' 53% BIAS!!! like OMG,,, no fridge wonder the back brakes don't work!!! 53%?!?!?!?!?!?!?!!!!!!

so... theres, and most others, are adjustable are from 0 to about 65%.... so, the question of wether or not it is a proportioning valve it answered, as well as the question of wether to remove it or not and replace it with an adjustable one that can do the job correctly...


Now, the new question... in adjustable P-valves there are 2 different types, the standard linear type like at Wilwood... Or the Knee/split point adjustable unit like the one i referenced above.... Justin did not have an answer for that question.