Rear brake pressure? - Team Camaro Tech
Brakes, Suspension & Steering Conversion questions, Steering & Handling

 
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post #1 of 6 (permalink) Old Mar 13th, 02, 04:20 AM Thread Starter
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Can anyone tell me what the normal working pressure should be to the rear brakes. I have a manual disc brake m/c. The front brakes work great, but the rears will not hold 2000rpm(off the ground), and that's with a 3500 stall converter. These are 11" drums from a donor car. I have checked the pressure both at the m/c and right in front of the flex line at the rear axle. Applying 110lb (measured with bathroom scale) to the brake pedal, I get 600psi measured at the rear flex line. Is this anywhere close? There is no stock prop valve, but I do have a wilwood adjustable valve. The measurement was made with the valve in the wide open position. Thanks in advance for any advise/opinions. David


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David
68 SS Camaro (377CID, 11.633 @ 115mph)
2001 Track Champion, Modified & Trophy 1 Div.-Wall Dragway, TX.
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post #2 of 6 (permalink) Old Mar 13th, 02, 10:38 AM
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I'm not sure I can be of any help, other than bumping you back to the top...

How did you measure the pressure at the rear flex line? That pressure seems really high to me, I thought rear drums were limited to only several hundred psi.

Have you checked your adjustment of the brake shoes? you should have a slight drag of the shoes on the drums if adjusted correctly.

I'm curious now, hopefully someone will give us a clue!

Bill C.


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Bill C.
Colorado Springs, CO

68 Camaro... someday!
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post #3 of 6 (permalink) Old Mar 13th, 02, 11:50 AM
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David Pozzi
 
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With that pedal pressure your fronts will be at or over 1000 psi.
Even if the valve is backed off, it may still proportion. Unscrewed is wide open. The strength of the spring controls the rate of proportioning and cut in point is controlled by turning the screw.

Most of these valves have a spring inside and if there is still any pressure of the spring on the valve remaining when the adjuster is backed off, you can be too low to hold the car.
You would either have to modify the valve or use larger wheel cyls, or try a front drum wheel cyl if it will fit.
A 69-70 Biscane, bel air, Caprice, Impala, all are station wagons, NOT sedans - rear wheel cyl is 1" and might fit. You MIGHT have a 7/8" cyl in there now. Can you verify that?
David

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[This message has been edited by davidpozzi (edited 03-13-2002).]
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post #4 of 6 (permalink) Old Mar 14th, 02, 07:46 PM Thread Starter
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Sorry its taken me so long to respond, been really busy. Anyway, here's the info that I have... I measured the pressure by attaching a guage (with a couple of adapters) to the end of the hard line where it ends in front of the rear axle. David, the prop valve that I have is the one with the handle, and not the knob, but both probably work the same way. I tried it with the valve fully open, and then again with it in the "closed" position and only saw about 100psi reduction (from 600 to 500psi) in pressure. (measured with the same pedal pressure, note: used my wife's bathroom scales, so they could be 20lbs. light or so, hehe) Should the valve have more affect on the pressure than that? I did replace the "El Cheapo" shoes that I had on it, and that seemed to help, but haven't got to drive it on the street yet. May pull the prop valve and install a splice just to check it. Thanks for the responses!

PS- Oh yea, the wheel cylinders are 15/16. I've thought real seriously about using the front cylinders, but can't seem to find the correct fitting to make it work. Guess I could use the regular flex line and then go from there?


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David
68 SS Camaro (377CID, 11.633 @ 115mph)
2001 Track Champion, Modified & Trophy 1 Div.-Wall Dragway, TX.

[This message has been edited by LoneStar68 (edited 03-14-2002).]
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post #5 of 6 (permalink) Old Mar 14th, 02, 08:59 PM
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David Pozzi
 
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Station wagon or pickup rear cyls would be bigger and work better.
That prop valve is probably still limiting your rear pressure, I think. There should be some adjustment on it besides that handle, a screw or threaded adjustment.

You can easily read rear pressure by connecting to the bleeder screw.

Get a piece of brake line at the store that has a bubble flare on it like the british cars use. plug your guage in to the other end and screw the tubeing into the bleeder hole.
Get the bubble tube with the long threaded end on it and you may have to grind off 1/8" of threads to make it bottom correctly.
Or just weld an 1/8" pipe thread fitting to an extra bleeder screw and drill straight through it, plug the side holes and you are set.
You can then tap into any bleeder screw hole.
I have a fancier pair of guages that uses a grease gun hose to extend the guage and a small T wtih a needle valve to easily bleed the hose of air.
David

------------------
Check my web page for First Gen Camaro suspension info:
David's Motorsports page
First Gen Suspension Page
67 RS 327 Origonal owner. 69 Camaro Vintage Racer, 65 Lola T-70 Chev SB Can-Am Vintage Racer

[This message has been edited by davidpozzi (edited 03-14-2002).]
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post #6 of 6 (permalink) Old Mar 17th, 02, 06:19 PM Thread Starter
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David, once again, thanks for the great info. Never thought of tapping into the bleeder for a pressure reading. I'll try using an old bleeder and welding on a fitting. Not much choice in brake lines in my neck of the woods, so probably wouldn't be able to find the bubble end line. Thanks again.


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David
68 SS Camaro (377CID, 11.633 @ 115mph)
2001 Track Champion, Modified & Trophy 1 Div.-Wall Dragway, TX.
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