Brake line pressures

[Skip Fix]

What is "normal" pressure when you are on the pedal for a manual system?

Current system for drag car 79 Camaro manual Vette deep recess 1" MC pedal in "top" hole 6.71 ratio. Front brakes 10.75 rotor Wilwood Dynalites 4 1.75 pistons. Hurst Line loc used as the T for the fronts and the port for the pressure gauge. Rear 79-81 WS6 TA 11.125 rotors with 2.5 piston "metric" front calipers(no ratcheting E brake calipers). Old Mopar adjustable prop valve in line-pressure port after that.

Car not running brake system all new just rolling in garage setting it all up.Bled the crap out of it, bench, gravity and pressure bleeder.Inner and outer Wilwood bleeders and bleeder on pressure gauge.

Problem has been alot of pedal travel to me, and a fairly easy soft pedal. Do see pads work and stops hand spinning front wheel. May need a larger MC since pretty big rear pistons.

Got the rear pressure gauge hooked up-didn't have enough line for the other one. With the prop valve all the way open less pedal feel but gets 550 psi at the bottom of travel holds it. Prop valve all the way decreased 400, a little better feel but still soft. Takes 3" of the 5-6" travel to even get 200 psi.Drops quickly with letting off pedal just a little. Seems a little low to me but never used a gauge before.

More line on the way to hook fronts up.

So anyone used a pressure gauge?

I know there is a 1 1/8 Vette MC. There also is the generic aftermarket 1 1/16 Mopar style that needs the bolt spacing massaged. At one time I saw a guy using a manual quick take up S-10 MC but it had a shallow rod recess.

[wmhjr]

My wilwood gauge only goes to 1500psi. My Wilwood brakes (7/8" master) give me 1400psi with a little effort on the front, and about 1100 on the rear. I could be wrong, but even 550psi just ain't even close to being enough.

[Skip Fix]

So what to look at MC not generating pressure?

[David Pozzi]

I'd expect to see 1300psi on the front on a 70's single piston front caliper.
Your 1 3/4" front calipers are a little small, should be closer to 1 7/8" or 1 15/16".

[Skip Fix]

David thanks for answering-most the Wilwood setups that use the Dynalites have the four 1.75 pistons on them. I think that is the largest for that caliper. They have been the standard four piston Wilwood front caliper in a drag or street kit until recently and are on their 12.18 kits also. I think just year they developed a Superlite caliper upgrade for their kits that has a larger pad and pistons that Frank and some of the others started carrying.

I was expecting to be seeing over 1000psi also. The 6' of line I got for both lines only made it to one running from drivers to passenger side where I had to mount the gauges to see them.More teflon line on the way by Wednesday to plumb the fronts to see where it is.

By the way I love old vintage road race cars like Lolas- I think I even have and old HO electric one somewhere. Made it to Laguna also when I was buying some Herb Adams stuff at the old shop on Cannery Row in '79-80.

[Skip Fix]

My math I get 9.62 sq In per front caliper and 4.9 on the rear calipers a side.

Factory 2 15/16 single piston 6.73 sq in.

So larger Sq in than even the factory 4WD that used a 1 1/8 MC.

[rrunner68]

Without knowing the amount of force you are pushing onto the pedal, its not very helpful to say you have XXXpsi at the pedal. For example, at 100psi input, I should see around 700 at the caliper. Its the M/C bore to piston ratio multiplied by your pedal ratio and psi of input at the pedal.

[wmhjr]

Without knowing the amount of force you are pushing onto the pedal, its not very helpful to say you have XXXpsi at the pedal. For example, at 100psi input, I should see around 700 at the caliper. Its the M/C bore to piston ratio multiplied by your pedal ratio and psi of input at the pedal.

I assumed the numbers were psi at the caliper. That's what mine were.

[rrunner68]

I assumed the numbers were psi at the caliper. That's what mine were.

You are correct, I mis-typed that line.

"Without knowing the amount of force you are pushing onto the pedal, its not very helpful to say you have XXXpsi at the *caliper*."

[Skip Fix]

Since I don't have a calibrated leg hard to tell what leg force has been a few years since I powerlifted and squatted 380 lbs as a 148.75 lber.The 500 psi was stomped as hard as I can at the bottom of travel and pumped the pedal a few times and the max pressure I could generate.

I would assume the pressure in line past the prop valve should be the same line pressure at the caliper since there are no restrictions to the caliper from there. So how do you plumb a gauge at the caliper-use the bleeder hole and T it so you can still bleed the gauge? Or T the supply line to the caliper?

[wmhjr]

I removed the bleeder and screw in the wilwood gauge. Pretty simple even for me.

[Skip Fix]

These are dual gauges set up to be used while running for changing brake bias on the run.

http://www.summitracing.com/parts/LNG-44136/

They now have some smaller ones that would have been an easier fit in the dash.

Since many calipers have different bleeder threads(heck my 78 TA the factory rear wheel cylinders were different than the replacement ones) does it come with a bunch of adapters? Still have a bleed to get the air out from under the gauge?

[Apogee]

Front Caliper Effective Piston Area = 4.80 sq inches w/ 10.75 inch diameter rotor
Rear Caliper Effective Piston Area = 4.90 sq inches w/ 11.125 inch diameter rotor

Aside from the obvious mismatch front to rear with the calipers combined with the fact that the front rotors are smaller than the rears, the pressure issue should strictly be a function of the input force, pedal ratio and master cylinder bore size. Brake torque will take the caliper piston areas, effective moment arm of the rotor and the pad coefficient of friction, but that's not what you're asking if I'm not mistaken.

You mentioned that you were running the 2.5" piston diameter, metric front calipers from presumably a G-body or 3rd-gen F-body, which may be a large part of the pedal travel issue you're having. The 1978+ metric calipers are a low-drag caliper design intended to be used with a quick take-up master cylinder. That combined with any air in the system could definitely lead to a pedal like you described. Did you confirm that your master cylinder was fully bench bleed? Given your MC bore size and output pressures, I would expect your pedal to be high and firm...I think that I would lean towards a smaller bore MC to get your pressures up and a quick take-up design to deal with the volume issues introduced by your rear calipers. There are alternative ways to constrain your pushrod if needed.

Tobin
KORE3

[Skip Fix]

Tobin thanks. The math in the front is still off to my brain. Four 1.75 pistons. pi x r x r x 4.3.1416 x .875 x .875 =2.405 x 4= 9.6. Or do you just use one side of the multiple pistons?

The factory TA 4WD used a 2 15/16 front with an "11" rotor that was a hair smaller and a the 11/.125 rear rotor and a 2.5 rear caliper. 79-80 had a cast iron non quick take up MC and 81 used the aluminum quick take up 1 1/8. With their dual diaphragm booster they used even less ratio than the regular power disc brake pedal ratio that was 3.91.

Right now just trying to diagnose the poor pedal feel, Bench bled util no bubbles. Pressure bled all 4 corners multiple times. Currently have Wilwoods BP-10 pads on front and some old new organic Bendix on the rear. The car will have skinnies on the front and a 9 or 10" slick on the rear. Estimate 3500lbs and guessing a 53%+ front bias on the weight.

[Apogee]

Skip, your PM Inbox is full.

You're correct that you only use half the number of pistons for the effective area calculation when looking at a fixed mount caliper. I know that it is counterintuitive, but just go with it. As for the master cylinder, no more bubbles doesn't necessarily mean no more air inside...just no more air coming out. I always plug both outlet ports and push on the piston to confirm that there is no more air hiding in the MC. A fully bleed MC will hydraulically lock after about 1/8" of piston travel, just enough for the seals to clear the compensator ports to the reservoirs.

Tobin
KORE3

[Skip Fix]

Thanks again Tobin. So how would you get more air out if you've bench bled and pressure bled already?

I might get some plugs and give it a try. I still think it is a MC issue either a bad one or air in there. With air I would expect the pressure reading to be all over the place and not as consistent.

"You're correct that you only use half the number of pistons for the effective area calculation when looking at a fixed mount caliper." Seems then like a fixed caliper shouldn't work as well as floating then to cram pistons in.

http://www.wilwood.com/Calipers/Cali...x?subname=D154 Single and Dual Piston Floater

Here are the Wilwood dual piston calipers that would swap on the rear. Might have to look at the dual 1.12 ones. But heck that is almost 3/4 the price of a Dynalite rear set up, but I'd have to pull the axles out to get the caliper bracket off.

[John Wright]

What Tobin is saying that sometimes the air gets trapped...it might just moved back and forth and not actually get bled out with the fluid as you pump the MC.....so while bench bleeding, run lots of fluid out of it until you can get it to hydra-lock.

[opnwide]

I think the 1" MC is marginal for this system. The 4 disc TAs came with 1 1/8" bore MCs. I've got the s-10 rear disc conversion (off a 2000 extreme) on my trans am, and I still have quite a bit of travel, even with the proper MC. A rebuilt MC from Autozone will run you $25. Give it a try. The 1981 version is a little more pricey, but has a plastic reservoir and looks like the MCs from the 1980-1990s. I cannot lock up my brakes (crappy pads and not much vaccuum assist) but the car stops well enough.

[Skip Fix]

So MC questions(still waiting for UPS to make the front line) on the Vette and most GM masters there is a 1/2" and a 9/16 fitting which is front or rear? Does is matter if both reservoirs are the same size-ie do the internal pistons do any metering? The Vette as well as an extra 81 TA MC have the smaller one in the rear. My TAs are over at the stirgae space across town. The 81 also looks like the reservoirs are pretty close in size

Second question I do have a Strange 1 1/16 MC new I could try. The reservoirs are very different sizes-to me unusual for one that is used on race cars with 4WD and ususally the same front and rear calipers.So on my setup which should get the larger reservoir? The 81 TA 1 1/8 has a pretty shallow hole to keep the pushrod in, the Strange isn't real deep like the Vette ones are. But heck the manual 67 GTO MC I bought hardly has a recess either.

Greg you know Steve Coombs over there in Austin?

[Skip Fix]

UPS came, got the line done, and have 1050psi on the front when I stomp on it hard-150lbs+ leg pressure. Rear is still only around 550 max. Both go to 400(a little past the acel pedal) at the same rate and numbers then the front I have plumbed to the front larger port leaves the rear in pressure untgil the very last stomp does it move off 400 to 550. I would not think the adjustable prop valve would cut it down that much. With a disc drum and the same prop valve I had it balanced pretty good for autocrossing. So MC stil not generating 1400 psi(max on the gauge).

So could it be the big rear pistons are taking so much volume on the small bore MC it can't build full pressure like it can on the front? A QT MC on the rear?

I guess I can try the line loc now and see if it holds pressure.

Greg I was drilling teeth yesterday, just a small mouth than you are used to a 6.5 lb cat.