Rear brakes barely working

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Hi guys,

I have a 2001 Mitsubishi Mirage that I am using as a track car.

It has been on a brake roller test and the brake bias is 91F/9R which doesn't sound right.

The brakes have been upgraded including
Different BMC and booster
Larger twin pot front calipers (from single pot)
Drum to disc rear conversion


At first I suspected (as is often the case) that the brake prop valve is designed for drum rear brakes and swapping that for one from a rear disc equipped vehicle would rectify this but after some research the part numbers between cars that have discs and ones with drums at the back are the same.

My next action is to open up the rear calipers to make sure nothing is stuck (seeing as these came off of a wreck with unknown history) but I don't expect to find an issue.


Any ideas of what could this be?
All lines seems fine.

Also is there a way to bench test a prop valve to see what the bias is?
 
I am not sure it is the brake prop valve, and judging by the fact that the same part number is used in other cars with disc rear brakes it suggests that either mine is faulty or not to blame
 
You have also put on much better front brakes. So naturally the balance shifts to the front. On a race car you would really want an adjustable proportioning valve.
 
Does that proportioning valve have a bleeder on it?

The one on my Toyota truck does, and it's the last to bleed when bleeding the brakes. Recently, I had to replace the valve on the truck with a new factory part, but if it fails again I am going with aftermarket because it's a lot cheaper.

I still have not fine tuned the valve for rear brake pressure. Going to pick up a brake pressure gauge next month to see what is what. Already busted the budget this month with steering parts and the valve, so the gauge will have to wait.
 
Here is my 2 cents.

What mk378 said plus your proportioning valve was designed for the original front calipers.

My solution would be either to install an adjustable prop. valve or as some people would do, experiment with different rear brake pads (more agressive). All these after you've made sure your rear calipers are funtioning properly.
 
Thanks guys,

I should have mentioned the upgraded brakes are identical to an FTO and as above the prop valve replacement part numbers are the same between my stock brakes and an FTO.
 
I'm thinking brake lines, you did install an OEM(or OES supplier like Aisin) master cylinder? Did Mitsu use a bigger MC on the upgraded cars and/or different ABS software?
 
Yes the BMC is OEM and was upgraded to one that came with those brakes.
It is 7/8" but I will be going 15/16" inch most likely.

The ABS equipped cars had different prop valves, 1" BMC and a larger booster
 
Originally Posted By: Spetz
Yes the BMC is OEM and was upgraded to one that came with those brakes.
It is 7/8" but I will be going 15/16" inch most likely.

The ABS equipped cars had different prop valves, 1" BMC and a larger booster

I would try getting the ABS prop valve and the bigger MC.
 
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