Is my brake master cylinder dead?

[1986.5supra_kid]

OK so i took my car to a shop because it was pulling to the left and i noticed that there was alot of brake dust on the left wheel. so i had a shop look over the brakes. they recommend my fluid be replaced because it was old or dirty looking. so they did this and now they told me that i need a master cylinder.

i drove it home with only 1-2'' of actual brake pressure (flame away i had no other ride home and i drove like your grandma and kept a respectable distance from the other drivers)

so i figured they just did a bad job of bleeding. so i had a person pump the pedal and hold it while i cracked the bleeder and my front brakes bleed perfectly but the rears had VERY little fluid and lots of air. like i had to turn it out like a whole turn to see anything happen.

maybe they messed up my master, it HAD perfect pedal travel and pressure before the shop visit.

symptoms
-engine off, pedal pressure at halfway
-engine on,pedal pressure,just inches from the floor
-no air out of the front brakes
-no air out of the proportioning valve
-lots of air out of the rear brakes
-no leaks found

thanks

 

[Nick M]

So....is there still air in the rear? The rear is bled before the front. In fact, the right rear is bled first, then work your way to the left rear, right front, and finally, left front.

 

[OneJArpus]

I'd say rebleed if it still happens it could be that the master is bad. I had this happen to me.

 

[OneJArpus]

I had 0 brake pressure after a while and air was escaping the seals inside of the master and the fluid would be pushed from the back piston to the front piston but would not travel through the lines

 

[Maple191]

Master cylinders can let go during a bleeding process if theyre on the way out. Just cause it drove in fine doesnt mean it was in good condition. Remember its just a rubber O-ring allowing pressure to build up. If its old its gonna tear. The actual pumping action is the cause of the wear. So you could immagine if you needed to slam on the brakes in an emergency.

 

[Suprapowaz!(2)]

Two seals inside the MC. Take off the MC, disassemble it, inspect those seals. If bad buy another MC and replace it. Re-bleed and you should be good.
You need to look into the caliper as well and make sure it's not sticking.

 

[Grandavi]

I would bleed the lines first though. (prior to rebuilding the MC or replacing)
Using a line with a clean bottle works very well if there is a lot of air. Always bleed the farthest distance from the master cylinder first.

Faulty MC should be identified by loss of fluid wouldn't it? (never had a faulty MC.. so I am wondering if that would be a sign... I know my clutch MC leaked horribly)

Curious where OP ends up with this... because I would have big issues if I went to bleed the rear myself and it had air in it after a shop dealt with it.

 

[Maple191]

Faulty MC should be identified by loss of fluid wouldn't it?

If its leaking externally then yes. If the seals are blown on the inside then no. You just wont build proper pressure.

 

[1986.5supra_kid]

picked up a MC from a junkyard and bled the lines. ive got better pedal pressure now but its not perfect. i think i still have some air somewere. the pedal travels 10-12mm before it reaches resistance. where as the old one before it failed was right at the top of the pedal 3-5mm. i have a new one on the way just incase this one fails in the future.

also changed the rotors,pads

 

[Poodles]

So....is there still air in the rear? The rear is bled before the front. In fact, the right rear is bled first, then work your way to the left rear, right front, and finally, left front.

Depends on your method of bleeding. If you're using a vacuum bleeder, they recommend going from closest to furthest.

To check if the master is dead, loosen the 4 bolts holding the master to the booster, and pull the master away from the booster. There's a paper gasket there and if the master does leak, the fluid has nowhere to go except into the booster.

 

[1986.5supra_kid]

the master was leaking from seal to seal internally. the bore was scored and the seals looked cut up.

 

[Nick M]

Depends on your method of bleeding.

I am guessing he isn't in a shop.

 

[Poodles]

I am guessing he isn't in a shop.

Maybe true, but a vacuum bleeder is cheap

 

[Poodles]

I am guessing he isn't in a shop.

Maybe true, but a vacuum bleeder is cheap