wishIhadanFC

So i converted to 4 piston calipers on my 91 coupe the other day, however my pedal feels really soft. I've bled the brakes about 3 times already and i can say with good certainty that there is no air in the system. I even bled the master cyl just incase (never let the resovior run dry though.)

I've seen threads where people talk about needing to use the booster and mc from the 4 piston donor car, is this the case for my 91?

Icemark

It is recommended but not required.

Carzy Driver

I swapped over when I went 5-lug, but that's only because the old brake booster was all rusty.

87nafc3s

you dont need a new master cylinder... same happened with my 5lug swap... there a valve or something like that that you need to replace, I'll check with my buddy that did exactly what it was....

coldfire

iTrader: (1)

it is not necessary, and chances are if your pedal feel is really soft you have other issues. be sure to PROPERLY bleed the brakes. let it sit overnight for any air to accumalate. also, remember that in the rear there are two bleed valves and they must be used in a certain order and situation (i forget what exactly).

wishIhadanFC

2 bleeders per caliper in the rear? I was curious about that because the bleeders face down which is not typical, since air would be at the highest point in the system.

87nafc3s - let me know when you figure out what valve it is.

uRizen

iTrader: (2)

^^ If you're replacing the rear caliper and it's been dry of fluid you use the bottom one, bleed the whole system, then go back and use the top one. If you've just replaced a line in the rear you only need to use the top one.

BTW, what's the recommended way of clearing out old fluid? The FSM says to just open the bleeders at all four corners and pump the brakes until nothing comes out, then fill and rebleed the whole system.

I've been considering this because I'm replacing my master cylinder this weekend and would like to fill it with a higher calibre fluid since I'm going to start taking it to the track and doing solo events. Spongy or boiling fluid on a hard braking corner = heartattack.

KingCobraV9

iTrader: (2)

i have a spongy pedal too after my swap. i even changed the proportioning valve and the booster and it still feels like crud. so i dunno what it could be either.

coldfire

iTrader: (1)

be sure to NOT pump the brake pedal when the system is dry otherwise you run the risk of damaging a seal in the master cylinder. so if you are trying to clear the whole system of fluid make sure you stop before it is all gone.

classicauto

iTrader: (3)

Get a vaccum bleeder

This way you don't run the entire system dry....but its time consuming and REALLY not required in most cases, unless you're upgrading to a DOT 4 fluid or something.....

speed_monkey

proportioning valves

87nafc3s

yup thats it... the proprtioning valve on my original brake system could supply enough throuput to put my tII brakes to any use... so we changed the valve and they work fine now... thanks speed monkey, it was on the tip of my tongue...lol