We have had a similar problem on our FP SCCA spitfire.
WE went through all the calculations and it suggested we were getting only
about 15% breaking on the rear wheels. Theoretically for our weight
distribution we should have had 40%. We then generated a 6 foot torque
wrench measured the brake torque at a constant pedal pressure. Pretty close
to the calculated value. The calculations shoed that we should go to 3/4
rear wheel cylinders. We have GT 6 drums on the back and GT 6 disks on the
front with Tilton dual 5/8 masters with a balance bar. The larger rear wheel
cylinders generated the desired 40% rear braking bias as calculated and the
brakes are much better in racing. It is important to lock up the fronts
first. But we could not make the back lock up at all with the 5/6 rear wheel
cylinders. In fact we could turn the rear wheel by hand.
----------
>From: Chris DeStaffany <destaff@yahoo.com>
>To: spitfires@autox.team.net
>Subject: Weak rear brakes
>Date: Sat, Jan 20, 2001, 2:18 PM
>
>
>My 1975 spit has weak rear brakes. If I drive it on
>gravel, I can relatively easily lock up the front but
>no amount of pedal locks up the rears. I have
>replaced the rotors and pads on the front. Replaced
>the rear shoes and turned the drums. I have rebuilt
>the master cylinder and pdwa. I have bleed and
>adjusted the brakes many times. I have looked at the
>rears over and over and can't seem to find anything
>wrong. The cylinder moves and the adjuster seems to be
>working properly.
>
>The parking brake does not hold very well, but I'm
>thinking that that may be a streetched cable. Anyone
>out there got any ideas. I'm a little stumped right
>now.
>
>Chris DeStaffany
>
>ps I recently got help from the list rebuilding my
>PDWA. With help, I was able to determine that the
>switch was bad and now it works properly.
>Yahoo! Auctions - Buy the things you want at great prices.
>http://auctions.yahoo.com/
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