4 wheel disk conversion

[monty73741]

i was watching tv...& they had a conversion of a civic from rear drums to disk

they replaced the brake balancer, i was thinking is the I4 break balance & the v6 balancer the same

[Yoda]

No they are different. Disc and drum brakes require different fluid volumes and pressure to operate correctly. Without changing the master cylinder and the fluid distribution block you are getting much less braking force from the rear brakes. The rear brakes usually 20% of the total braking force whether it be disc or drums. We did a brake dyno test ones and a disc brake conversion with out the correct fuild distribution system was a marginal pass compared to a the original drums and the corrected disc brake system when compared to the OE specs.

[Yoda]

The other night I saw a kid that spun his car in the rain and punched it hard into a guard rail. Turns out that it might have been caused by him converting the rear end to discs from drums and not converting the rest of the system that caused an initial braking instability causing the rear end to come around on the rain soaked road surface. Of course it didn't help that the car was lowered and the alignment wasn't corrected so bump steer and camber wear on the tires also became a factor. Since everyone knows the difference mechanically in the braking system between a Disc/ Drum and a Disc/ Disc disk but when it come to the hydraulics the that operated the brakes everbody pretends that they are the same and maybe the brakes will work properly when the pedal is depressed. This kid found out the hard way and the cop on the scene was also very intrested in my mechanical observation of the accident in his report. I'm sure the kids insurance company will go over the car as well too when they see the police report.

What are the exact difference in the braking system between a rear drum brake and a disc brake system?

The master cylinder for a Front disc/ rear drum brake system is smaller and displaces less fluid volume as the rear brake cylinders are much smaller than the a disc brake caliper piston so the total system needs less fluid volume to function than a disc/ disc system.

The proportioning valve is also different for the same reason as it will send more fluid volume to the front as the rear drums need much less fluid to achieve maximum braking force

A drum brake system has a meter valve or what is some time called a "hold off" valve. The purpose of this valve is to delay full pressure being sent to the front calipers, allow the pressure to the rear drums to rise enough to defeat the tension of the shoe springs. This is done to balance front to rear braking force and keep the rear-end stable when braking on slippery road conditions.

The last item that is different in a rear drum brake system is the residual pressure valve. This valve maintain about 10PSI of pressure to the rear wheel cylinders. A rear disc system will only need to hold 2psi of line pressure. The purpose of the valve is to maintain a little line pressure to the rear brakes to remove any spongy pedal feel or the need to pump the brakes. If a drum brake residual pressure valve is used with a rear disc conversion the will mean that there will always be braking force applied to the rear calipers

What does all this mean if you've only swapped out the rear brake mechanics from drums to discs without swapping all the hydraulic system as well? First you will loose power to frictional looses due to the rear brakes will always be dragging because of the drum brake residual pressure valve will retain 5 times more line pressure that necessary for disc brake caliper. There will be a false sense of improved braking capability first due to the believe that the rear discs are better that the drum brakes even though the hydraulic system is not capable of operation the rear calipers to any where near the same braking force as the original drum brakes added to this is the additional residual pressure to the rear calipers will give the impression of the braking system feeling firmer as a whole. Given the right conditions the combination of the drum brakes residual pressure valve and "hold off" valve could cause the rear end of the car to become unstable if the road condition are marginal especially since most guys that would be swapping the brakes would probably already lowered the car and not corrected the wheel alignment or anti-roll bar geometry either. The combination of bump steer, which many mistake for increased handling and the brakes, now out of balance rear to front when the brakes are initially applied, can cause the rear of the car to be come unglued. Normally the rear brakes does about 20% of the total braking from the factory. Between the smaller master cylinder and differences in the proportioning valve the conversion to rear discs with a drum brake hydraulic system will generate less maximum braking force to the rear but could potentially take away braking force to the front as well if the proportioning valve can divert fluid volume and pressure to the rear calipers as well. Again there will be a faults sense of improved braking as the brakes are initially applied but will fall off as more force is needed when compare to a complete correctly engineered disc/disc braking system or even the original disc/drum braking system.

[nobodyhere671]

what do i need, to convert my mx-3 rs to rear discs properly?

[cyclonekid78]

For my swap. I used the GS rear spindles with calipers. You will need the brake booster, brake master cylinder ( with the brake lines to proportioning valve) , proportioning valve with front brake lines. Also need the ebrake cable too

[wytbishop]

Yoda,

Am I correct in my assessment of the function of the proportioning valve? I assume that the valve diverts pressure to the rear brakes after a certain pressure is present in the front brake circuits. I am trying to think of any other possible cause for my current inability to bleed my new rear calipers. I believe that the valve is corroded inside and not allowing fluid to be transferred to the rear. Mazda wants $142CDN for the valve. I will try one from the wreckers first, but it's not simple to confirm the functionality of a used one. I would have to hook it up to pressure, pressure it up and confirm that fluid flows to the front ports, then plug the fronts and pressure it up and see fluid out the rear ports...PITA.

The only other cause I can imagine is if both the rear lines are pinched somewhere. I see the odds of this being quite low.

Am I correct as far as you know in the operation of the proportioning valve?

thanks,
Charlie