[Fot] Rear brake help needed

Fri May 7 14:20:03 MDT 2021

Like with any brake related issues , you need to work on facts .
* line pressure front and rear
* actual Temperature front and rear ( temp paint is best).

Are you locking rear brakes at all ?? My first inclination is , the rears are dragging -

Wrong geometry at balance bar ?
Master cylinder free play ?

Good luck
T

Sent from my iPhone

On May 7, 2021, at 1:01 PM, Greg Blake via Fot <fot at autox.team.net> wrote:

Fellow racers and breakers of parts,

This past weekend we had wonderful racing at Eagles Canyon Raceway.  Ownership turned over a couple years ago and they have finished a multi-million dollar renovation of the track, it is truly amazing now.  ECR has always been high on my list because of its potential with the natural terrain, but the lack of good construction methods had held it back since it opened.  That has all been corrected now and it is truly amazing…well enough on that for now.

My problem
For the past two years I have not been able to finish a race weekend w/o completely destroying my rear brakes.  Usually related to heat build up that manifests as a blown wheel cylinder or brake shoe linings that start cracking and chuncking off the shoe causing further rapid wear.  This last weekend, I had a total rapid failure of the rear brakes when the right rear cylinder let go under hard braking from 100+ mph, I may have blown another seal in the drivers compartment when that happened, but will spare those details.  Luckily, my dual masters allowed me to get the car slowed reasonably with just the front brakes.  For any of you that have had this happen before, you understand that with a balance bar setup, you don’t end up with full brakes when one end fails.  The balance bar rotates significantly to the side that has zero resistance.  In my case I’d estimate that I had about 20% of my front brakes available to slow the car, like I said, interesting experience that I’d rather not repeat again.

My setup
Twin Wilwood 0.7 master cylinders with a custom balance bar.  Stock front TR6 calipers and stock 9” TR drums with 0.75 wheel cylinders (I’ve tried everything from 5/8 up to 7/8”).  I also run a hydraulic proportioning valve to be able to fine tune the rear hydraulic pressure.  I was running with the aluminum rear drums until I destroyed the linings last year in one of the incidents when the rear shoe linings cracked and broke completely off allowing the shoe to cut groves into the drum linings. My routine setup is to put the car on jack stands to adjust the brakes.  I start by running the rear drum shoe adjusters in until I perceive slight drag, then back off two clicks.  Then I adjust the rear mechanical bias with the balance bar until I have the front and rear brakes engaging (dragging) at approximately the same point of pedal travel.  I then fine tune the adjustment to dial back the rear bias until I can threshold brake at speed and lock the fronts slightly before I lock the rears.  This allows me to trail brake with confidence that I’m not going to swap ends because of too much rear brake.

I have used stock, portifield, and TSI drum linings all with the same results.

I’m looking for a couple things from the group (no disc conversion ideas please):

  1.  What is wrong with my rear brake setup and adjustment approach?
  2.  What is your setup and adjustment approach?
  3.  Do you like the amount of rear brake you get with your approach or is it a compromise to keep the brakes from failing?
  4.  Lastly, I hate our wheel cylinders, its is a terrible design and I think we have real junk on the market today.  I have a theory that the sliding wheel cylinder assemble is part of the root cause of my failures, along with heat.  The piston and seal are dealing with some severe side loading once the cylinder starts sliding around to do its job and I don’t think it is up to the task, especially what we get these days.  Has anyone modified their backing plates and shoes to use a modern day twin piston wheel cylinder that is bolted to the backing plate?  Or has anyone ditched the TR drums all together for another 9” or 10” drum backing plate and guts from another source.  I’m open to all drum brake ideas at this point.  I’m really tired of not racing because of rear brake failures.

Sorry for the long post and thanks in advance for your ideas.

Greg
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