I want to install 12" rotors and PBR calipers on all 4 corners of my 72 Cutlass; parts will come from an '89 C4 corvette. I'm installing 17" AmRacing VN500 rims (they look like olds SSIs) so rotor clearance won't be an issue. The Vette came with a 7/8" mstr cylinder and the Cutlass has a 1-1/8" mstr cylinder. My biggest concerns (other than whether or not it will stop) are excessive pedal travel and getting enough brake line pressure to stop safely.

I can use a 1" mstr cylinder from a '75 Vette (disc/disc) and use a disc/disc proportioning valve from a '79 WS6 TransAm to properly set the front-to-rear brake bias. The 1" MC has 31% more piston area than the 7/8" MC from the C4 Vette; which means 31% more fluid volume and the pedal will travel 31% less, applying the brakes at a higher point of pedal travel. But with the stock a-body brake booster the brake line pressure will be about 20% less than the C4 Vette brake system.

If I move the booster push rod to the upper hole (typically used for manual brakes) the pedal will travel 8% further (applying the brakes at a lower point of pedal travel) but the line pressure will be 26% higher than the C4 Vette system.

Which approach is the better one? I really hate doing things twice so I'm trying to do as much homework upfront before making assembling the parts and making the swap. I'm looking for constructive criticism and feedback. Thanks!

 

Are you sure you are using C4 stuff. C5 is what most people are using.

 

It's all C4 corvette stuff. I started collecting parts about 9mo ago. The goal was to fit larger than stock brakes inside 15" SSIII rally wheels, and the C4 rotors and calipers were smaller than their C5/C6 counterparts. Unfortunately the center section of my 15" rally wheels is exactly the same size as a 14" rim, so the wheel kind of shrinks down in the middle. The C4 rotors are 12.01" in diameter and 2.19" high, which puts the caliper too close to the back side of the rim to clear the center section.

Even the Am Racing wheel in a 15" size is pretty close with the C4 parts. I could probably grind the cooling fins off the caliper or something, but is was too close for my comfort. So I decide to go with 17" rims for better brake clearance which will also allow me to run a more modern tire for better wet & dry traction, cornering, etc.

The other big deciding factor for me was the integral parking brake system on the C4 rear caliper. It is a simple lever mechanism that pushes the pads out against the rotor to set the parking brake. The C5 rear caliper uses a separate, mini brake shoe nestled inside the rear rotor hat area for a parking brake. This would require me to modify a C5 backing plate to mount to my '72 10-bolt differential. The C5 backing plate attaches with 3 bolts (not 4) whereas the 10-bolt axle tubes have a 4-bolt mounting plate.

This just sounded like a lot more work (and expense) than I wanted to get into, and I've already got to fab up caliper mounting brackets, front and rear.

 

Ok! I've already found a few errors in my number crunching. So I've created a spreadsheet to compile and present all the numbers in one place. I also ran a third scenario, drilling a new hole in the brake pedal arm between the power brake hole and the manual brake hole to give a mechanical ratio of 5:1. This should compensate for the additional fluid volume (and high brake pedal height) and increase the total line pressure as well. I also calculated the pressure at the brake pads (fronts only) to compare the systems from another perspective. I'm just trying to be thorough here!

Unfortunately this brings up a new question. The stock C4 system will generate 4070 lbs. of force at the pads. F=Press x Area. Whereas the stock Cutlass system generates 6534 lbs. of force at the pads. Should I be more concerned about the matching the line pressure of the stock C4 system, or should I focus on the force at the brake pads?

I'm trying to think of a catchy Yoda-speak reference to the Force, but I'm too brain dead to be that clever. LOL.

https://classicoldsmobile.com/forums...1&d=1378430231

 

I would skid the TA prop valve and get an adjustable one. They are set rather conservative from the factory, and it's valved for a 79 TA, not your Cutlass with Vette brakes. Vehicle weight, suspension, and tires all effect how the prop valve should operate.

Also, brake pedal height should not be a factor in your decision making process. If your booster doesn't use a threaded rod and clevis, it's easy enough to make it adjustable.

I know what you mean about the parking brake deal... I have C5 brakes on mine (kit from KORE3) and I don't have a parking brake yet because I wanted to save money at the time. This winter I'll have to go back and install the parking brake assembly, now that I have the 5-speed I don't trust just leaving it in gear when parked.

 

Mr.Nick:
I want to use a stock style proportioning valve to keep the brake warning switch. I thought the TA would be a reasonable match to the Cutlass. Also, I'm planning to mount the prop valve at the MC to allow more room for headers. What would be a good alternative to the TA valve? I've read that some valves have pressure "hold-offs" in the rear circuit to keep the brake shoes from retracting too far when you release the pedal. Maybe I should use a Vette valve (they're designed for 4-wheel disc brakes) but what year would be a good fit? And how do you set an adjustable prop valve, and where is the best place to install it? Thanks!

 

Just use a drum brake distribution block with the adjustable prop valve, just like the 67-70 cars did from the factory (well, except for the adjustable part...).

As for the M/C choice, definitely match the line pressure. Using a larger piston means that you need to press harder on the brake pedal to achieve the same line pressure. Now, if the brake booster can make up the difference, maybe there's no issue. I don't think you'll be able to easily get the pushrod into the upper hole on the brake pedal, as the manual brake M/C centerline is higher on the firewall than the brake booster centerline - the manual M/C uses the two upper bolts on the firewall, the booster is centered between the four firewall bolts. By the way, Wilwood sells a tandem M/C with 7/8" bore.

 

I have a Wilwood adjustable PV mounted on the frame rail about 6" behind front edge of drivers door. It's on a small flat bracket, so the **** sticks out a bit toward the transmission and is easy to reach.

As for setting it, check this out:
http://www.chevyhiperformance.com/te...tioning_valve/

Also read this post:
http://www.pro-touring.com/threads/1...stock-style-PV

 

Thanks for the info.
Joe: So a '70 distribution block does not have any metering or residual valving? It's just straight thru block with the switch in the middle? And will the 7/8" Wilwood MC mount to my stock booster? There seems to be 2 sizes for the backside of the factory master cylindes. I don't know the dimensions but the earlier MCs have a big "butt" and the later MCs have a smaller "butt" where they mount to the booster.

Nick: I don't know if I like the mounting the valve under the car. If it's not too large, could I mount it right next to the distribution block off the MC? I'll need to make a bracket for the distribution block anyway, might as well put the prop valve on the same bracket.

Rodney

 

You can mount it anywhere, in any position. You certainly can mount it right off the MC if you'd like. I probably would have done that too, but with the hydraulic clutch and line lock, it was getting crowded in that area.

 

Correct.

The Wilwood is supposed to have slotted holes where it bolts to the booster to fit both Ford and GM bold spacing. I can't speak to the diameter of the pilot that fits into the booster, however.

 

Here's a pic of the adjustable pushrod on my booster, just for reference. You can adjust pedal height this way.

 

I see how the adjustable clevis will help set the proper pedal height, but will that adjust for too much pedal travel if the MC bore is too big or the pedal ratio is incorrect?

 

I may have a clearance issue with the C4 rear calipers. The parking brake mechanism and cable stick out quite a lot off the back of the caliper and may hit the frame rail or shock mount when the suspension compresses. So, I did some internet research (how did guys do these kind of swaps before Google??) and it seems that a '98-02 Camaro LS has a rear disc set-up that is a direct bolt-on to a 10-bolt or 12-bolt axle flange. It has a "shoe in hat" style parking brake that is nested inside the rotor hat that will connect in the same general location as the stock parking brakes cable. This set-up gets rid of all the extra linkages and protrusions on the back of the C4 caliper.

Here's where the "too good to be true" bummer may kick in. The Camaro backing plate is about 1/4"-3/8" thick and mounts where the stock brake backing plate would go - see the pics below. The Olds uses bolt-in rear axles; whereas the Chevelle has a C-clip to keep the axles in place.

Here's the million dollar question: If I "sandwich" the LS backing plate between the axle tube flange and the axle retaining plate, I've just shimmed the retainer outward by about 1/4"-3/8". Won't this cause problems keeping the axles in place or seated into the differential carrier? It seems like the axles will be too loose and the bearings will slide in and out of the axle tube? Any thoughts?

 

I forgot to post a pic of the C4 caliper.

 

Here's the LS1 caliper with the "shoe in hat" style e-brake. It doesn't have all the linkages and brackets on the back for the e-brake like the C4 caliper.

 

Didn't realized you posted on this again. Did you make any progress?

 

I found a machine shop that would turn-down my rear axle flanges so the Vette C4 rotors will fit over the flanges. So I was finally able to trial fit all the parts. The Vette '88-96 C4 rear calipers have a parking brake lever arm that hangs off the back side of the rear calipers, and I've confirmed that the parking brake arm will hit the frame rails when the suspension compresses. (2nd pic)
So I'm now looking for some '98-02 Camaro LS rear calipers and backing plates which have a mini brake shoe inside the caliper hat to provide a parking brake. According to many people on the net, these are a direct bolt-on, but are designed for C-clip rear ends. The LS backing plate looks to be about 3/8' thick, whereas the stock, Olds backing plate is maybe 1/8". I'm working out how to compensate for the additional thickness of the LS backing plate so when the axles will be tight when they are bolted in.

 

Forgot to attached the pics.

 

Hmmm, I'm not 100% sure I know what mean about the backing plate causing slop in your axles. Wouldn't you put the bearing retainer on first, then the backing plate for the caliper and bolt it down?

I'll be in the garage this afternoon, I'll take a peek at mine and see how it works. Been a while since I did the brakes. Mine are C5 though, but might still provide some insight.

 

I think the LS1 backing plate will be "sandwiched" between the mounting flange on the axle tube and axle retainer - just like the stock drum brake backing plate. These LS1 backing plates are a cast part that is substantially thicker than the original drum brake plate. I'm talking with a guy on ebay about a used LS1 rear brake set up. Once I have parts in hand I can trial fit the parts to work out the installation questions. I saw some pics in a post on the Chevelle forum that showed 2 half circle spacer "rings" that fit inside the LS1 backing plate to keep the bearing and seal pressed into the axle tube when the retainer is bolted down. I may have to fabricate similar spacers for my application. It would be very helpful to see some pics of your rear disc set-up as the LS1 and Corvette rear brake systems are very similar, and some parts may even be interchangeable from what I've read on the F-body forums.

 

Update on my quest for rear disc brakes with a working parking brake. I purchased a set of used calipers, backing plates, brackets, and rotors off '98 Camaro LS1. Found it on ebay for $150 (included shipping the rotors) and thought it to be a good deal. The parts arrived Monday, in a very heavy box. The parts were mega dirty, but cleaned up nicely with a little degreaser and alot of elbow grease. I took the parts to the shop tonight for a trial fit. Here's what I learned.

1. I'm going to swap the LH and RH parts so the parking brake lever will pull towards the front of the car (like the stock drum brakes did) and there is a little more clearance mounting the caliper in trailing position instead of leading position as was stock on the Camaro. This should allow me to modify the stock parking brake cables and install them in roughly the same location as stock. I will need to make a bracket to hold the outer sheath of the cable, but I think I can bolt this to the front 2 axle flange bolts.

2. The LS1 caliper sits too far inboard to be centered properly on the rotor so I will need to install shims between the axle flange and the LS1 backing plate to push the caliper outward a bit.

3. I think I solved the axle retainer question. I read on-line that some rear disc kits include 2 "c" shaped spacers that install between the axle seal and the stock retainer plate to make up for the additional thickness of the LS1 backing plate and keeping the axle bearing tight inside the axle tube.

I'm super please with my purchase and think the set-up will work well very with my PBR C4 rotors and calipers that I plan to put on the front. I do have one issue that I'd like input from all the wise CO members: The LS1 rotors are drilled for 4.64" bolt circle and 1/2" wheel studs. They fit onto my stock axles with the 4.75" BC and 7/16" wheel studs, but there is some slop between the 1/2" bolt holes and the 7/16" studs (look at the last pic).

Do I need to worry about the gaps or will the rotor stay put once the wheels are mounted and the lug nuts are torqued down? I guess I could have a machine shop drill new holes in between the 1/2" holes on a 4.75" BC for 7/16" studs.

 

Where those rotors re-drilled for 1/2" studs on a different bolt pattern? I thought all the LS1 F-Bodies has 4.75" (120.65mm) bolt patterns. The new Camaro's, GTO's and G8's etc, have a 120mm bolt pattern, which is 4.72".

What is a 4.64" bolt pattern even for?

 

Nick:
I'm going off info from the RockAuto site. The Rock Auto website has been an invaluable resource to cross reference the specs of parts across many different years and models. Rock Auto has parts from Centric, Bendix and Raybestos that all list .64 hole size and a 118mm/4.65" bolt circle. See attached files.