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Author Topic:   Disc Brakes
Dirt Full Roller

Total posts: 33
posted April 18, 2005 08:22 AM  
We have just added disc brakes to our rear end. They continue to be poor and spongy. We removed the proportioning valve and bled. Think our master cylinder may be bad. Have a cast cylinder now, have heard the flat plastic type may work better. What are people running, do the check valves need to be removed from them? Thanks for any insight.

Dirt Maniac

Total posts: 148
posted April 18, 2005 03:47 PM  
Did you add rear discs to your car and plumb the existing master cylinder to them ?
If so you not only need a new master cylinder, you need one matched to the requirements of your system.
Is there a version of the car you are racing available with factory disc? if so that would be easy.
If not you need to do some learning and figuring.
Discs usually have a much bigger piston in the caliper requiring more fluid to move the fluid. If your car had discs in front and drums in the rear; look at the size of the reservoirs, the disc side is much bigger huh?
If you must run a stock master cylinder find one that is off an old vette with manual 4 wheel discs. You still may end up juggling stuff though.
2 handy things to remember. small piston master cylinder equals more pressure and less volume. Big piston gives less pressure but more volume.

Some People Race For A Living; I Live To Race

Dirt Full Roller

Total posts: 28
posted April 18, 2005 04:45 PM  
If you find the proublem let me know,I did the same thing,add disks on the rear same as the front,new master cylnder 1 1/16 bore,
by passed the rear part of the portion(sp) valve ,bled the heck out of it,
and they still stink!!

Dirt Freak

Total posts: 402
posted April 18, 2005 05:56 PM  
Make sure the caliper mounts are welded on straight. If they are on a little crooked they will twist caliper and push the piston back in. When you push the pedal the calipers have to take up the slack before they can squeeze rotor and the pedal will feel spongy. pull the wheels off and have someone look very close at the caliper on each side while you push the pedal down. If the calliper is crooked it will straighten out when pressure is applied. It should not move when pressure is applied.

What are you running for hoses? The first time I put disks on the back I just ordered new cheap rubber hoses and never did get a good pedal. Changed them out for braided lines and noticed a huge improvement.

Dirt Forum Champ
Total posts: 569
posted April 19, 2005 07:07 AM  
I had the same problem on my first go round with rear discs. Where are the calipers mounted? I mounted mine on the top of the disc and even though the bleeder was the highest point, I could never get them completely bled. Once I moved them to the back of the disc and that helped a ton. Also remember those brackets have to be perfectly in line with the rotors.

Another thing that helped was replacing the nylon brake lines with steel.

BTW - I'm using big GM Metric calipers on all 4 corners with a Wilwood dual master cylinders.


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