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The mkiv Supra Owners Club

Brake Bleeding Issue


Shilakadaddy

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I've hit a small problem while attempting to bleed my brakes, here's what happened...

 

I changed my front discs and pads yesterday, and figured while I was there, I may as well change the brake fluid too. So I got a mate round to sit in the car and press the pedal, while I loosen and tighten the brake bleed valve.

 

Starting at the passenger rear of the car, I loosened the valve and asked my mate to pump the pedal and keep topping up the master cylinder reservoir with fresh fluid. At first a small amount of air was released from the caliper along with the old fluid. After about half a litre of fluid was pumped out, a large amount of air started to come out as well - far more than you would expect to see given that my brakes worked fine previous to this! We checked the level on the master cylinder reservoir and that was well above the minimum and hadn't dropped too low at any point during the bleeding process. We tried to keep pumping the fluid through, but nothing was happening now - the fluid level stayed the same, and nothing else was getting pumped out of the caliper.

 

I came inside and had a search around on here to see if I could find a solution to the problem, and managed to find a lot of references to the brake master cylinder, some saying that the seals can get flipped if the pedal is pumped too fast. I tightened the bleed valve back up and pumped the brakes a few times to see if the pedal would hold its position - however it just went down to the floor - again suggesting an issue with the master cylinder.

 

I've since removed the master cylinder and taken it apart to see if I can see a problem, but it looks fine. I can't see how any of the seals could possibly be flipped and none of them are split or even worn.

 

I've taken some pictures but can't get them on the computer at the moment (lost the cable!) so does anyone have any pictures of how the internals of the master cylinder should look? My next course of action is to rebuild the master cylinder, bleed that, then try again. But before I do that, does anyone have any suggestions of what may be the problem? Another component that may have an air lock in it? Or something else I may have done wrong? :blink:

 

Any help appreciated! :)

 

Cheers, Chris

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Wow, a post with excellent grammar and no spelling mistakes, should this be a sticky? :)

 

 

Have you got UK or Jap spec brake calipers? UK ones can sometimes be a bit of a whatsit to bleed, in theory you need to force the ABS pump to constantly run whilst bleeding the system. In practice I rarely find this necessary. Talk of master cylinder or caliper seals "flipping" is total BS, they just can't do this. If you have J-Spec front calipers it's possible, and surprisingly easy, to crack the caliper casting around the bleed nipple if the valve is overtightened. This will result in a spongey pedal, but I would have thought the resulting leak would be quite obvious. Let me know more of the braking specification of the car, and I'll see if I can assist more specifically.

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Thanks Chris, I feel really proud of myself now! :)

 

Sorry, I forgot to mention, I have UK spec brake calipers. I couldn't see any fluid leaking from anywhere at the time, so I assume everything is still intact. I was using Halfords Dot 5.1 fluid, although not sure what was in previously.

 

Dave F, those pictures are great! As far as I can see, the internals of my brake master cylinder are exactly the same :)

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:blush: erm... no :blush:

 

I don't know why, but I just assumed I didn't need to do that when simply attempting to flush out the old fluid! :rolleyes: So would this explain my issue? :D

Just a bit:p

The link that Colin posted is a good guide, but i would add to that to apply and release the brake pedal quite slowly.

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Use a bleed tube with a non-return valve on it. Then no air can go back into the system when the pedal is lifted. You don't need a friend to assist either. Cost me £8 from Halfords. Make sure the bottle that the fluid is pumping into is higher than the bleed valve.

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Yes, it certainly sounds like the bleeding method was at fault. I am not a fan of bleed tubes as upon releasing the pedal air can be drawn into the caliper via the threads on the bleed nipple, I prefer the good old fashioned manually opening and closing the nipple, or pressure bleeding myself.

 

I didn't realise that, might do mine again using traditional techniques in that case:-)

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Cheers guys! I'm happy the problem was just me being dumb :D

 

I did have a clear hose attached to the bleed nipple, which was full of fluid at all times, so I assumed when the pedal was released, the only thing that could be sucked back through the nipple would be the fluid in the tube! But obviously not!

 

I was considering one of those pressure bleeding kits, but I've read mixed reviews about being able to get a good pressure seal on the master cylinder reservoir. Anyone got any experience with them? Might be a worthwhile investment for the future...

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