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On my 1990 mk3 turbo, It seems like the brake pedal goes soft while braking. From what I’ve read it sounds like a common problem, and it all points to replacing the master cylinder. Has anyone had this issue? And was the master the problem? Also, I’ve read horror stories of trying to bleed the brakes on these, are they really that bad!! Thanks in advance!
 

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1987 Supra Turbo R154 Pearl White, Blue Velour
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Just in general life it sounds like your master cylinder, irrespective of being a Supra. :)
 

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On my supra (not to say this is your issue) I had zero hydraulic pressure at 3 out of 4 wheels. So only my driver front wheel had working brakes. Came down to being a bad master cylinder (the seals internally in the master had failed, preventing pressure from building in 3/4 calipers. But seized up calipers can also cause a soft brake pedal (from my experience as a tech). I'd recommend taking some needle nose vice grips (4 of them actually) and slide some rubber hose over the needle grip parts(rubber hose will prevent the grips from chewing up rubber lines). Then used the vice grips to pinch off the soft rubber brake hose's going to each caliper.
You can start with pinching off one hose at a time to try and eliminate a possible sticking/seized caliper. When a brake line with a seized caliper is pinched off, the brake pedal will become stiff again.
So..
1.pinch a hose (that attaches directly to the caliper)
2. press brake pedal
3. brake pedal hard again?
a. yes? remove that wheel and check for seized caliper, caliper slides, rusted in place brake pads
b. no? remove vice grips and go to the next wheel
repeat this step with all 4 wheels or until your brake pedal feels normal.
this usually takes about 5-10 min to do for all 4 wheels.
If your brake pedal is still soft after checking all 4 wheels independently , then start pinching off all 4 lines as you might have multiple seized calipers. This is definitely not out of the question as these calipers are 30 years old now and those rubber seals don't last forever.
If after pinching off the hoses at all 4 wheels and brake pedal is still soft, you have 2 options.
1. Flush brake fluid (which is time consuming, but not difficult with a helper....actually if the car is equipped with ABS, those ABS pumps/modules can be tricky to bleed, but I have no experience with bleeding ABS pumps on a supra so I can't say for sure)
2. replace master cylinder and then flush brake fluid.
If it comes down to this, I'd honestly recommend just replacing the master cylinder so you won't have to worry about a 30 year old master cylinder failing after doing a 100mph pull.

On a side note, the rubber hoses themselves can also become swollen internally and cause a restriction in hydraulic fluid flow (brake fluid). So if a rubber hose is pinched off and the brake pedal becomes hard again, but you cannot find any seized bake parts on that wheel, the hose itself may be faulty/clogged.
 

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Discussion Starter #4
Thanks for all the info! I ordered a new master cylinder. I’ll start there and then go to the calipers. Good idea on the rubber hose over vice grips, I probably would’ve clamped down and chewed up the hoses
 

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If you don't use the rubber hose, the gribs wont completely destroy the line, but for aesthetics it's helpful. Also, bench bleed the master cylinder before installing it otherwise it'll be a pain to bleed the system completely if it's installed dry.
 

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1987 Supra Turbo R154 Pearl White, Blue Velour
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After my car being parked so many years and only moved in and around the driveway, I took it to my mechanic to check the brakes. He was like WTF? 2 of your cylinders are rusted up and you only have brakes on 2 wheels.

In the end, I had him totally rebuild each wheel. Rotor, caliper, pad, hoses, hardware, bearings. Stops on a dime now :p
 
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