A70 Brakes Don't Work, Please Help Me!

**gttwinturbo** · 15-12-2008, 06:16 PM

Okay so heres the story: I upgraded my front brake rotors on my GA70 from a 278mm rotor to 300mm rotors from a 89+ ma70, I only swapped the discs and caliper carriers NOT the calipers as they are the same. I did not open the hydraulic system in any way to do this. However after putting everything together the pedal was really low and sank to the floor so I bled the brakes but there was no difference, thinking that it was my master cylinder I had it refurbished as a replacement was unavailable but still the brakes were bad even after being bled twice by a reputable brake shop.The master cylinder was properly bench bled and the entire system has been bled multiple times. The final time I took it to the brake shop they clamped the two front brake hoses and the pedal was as it should be this lead them to tell me that my proportioning valve may be stuffed as it it the only other component between the master cylinder and the calipers which have been tested and are fine and there are no leaks anywhere. In conclusion can the proportioning valve go bad? and is this low, soft, sinking and generally crap brake pedal a result or symptom?

In short: - there is no air in the system including the master cylinder
- the master cylinder has just been refurbished and is in top condition
- the calipers, discs and pads are all fine

WHAT ELSE COULD THIS BE?

ANY help on this would be greatly appreciated as im starting to feel like it will never be fixed as I've been struggling with this for over a week!

**oldcorollas** · 15-12-2008, 08:51 PM

same thickness disc?
are the pistons retracting too much?
if you pump the brakes does it get better, or it slowly sinks to floor?
etc..

excessive pedal travel measn there is slack somewhere.. either as air in the pipes, or as mechanical slack where pistons are pulling back, or have to move too far.

**STR8 2.8** · 15-12-2008, 09:23 PM

seals on the pistons gone? as in fluid seeping past the piston in the caliper.

**allencr** · 16-12-2008, 05:35 AM

Originally Posted by gttwinturbo

...reputable brake shop. ...clamped the two front brake hoses and the pedal was as it should be ... tell me that my proportioning valve may be stuffed as it it the only other component between the master cylinder and the calipers...

Sorry, I don't know what an A70 is, but I'd be very surprised if clamping hoses was not a good test for isolating the calipers and that this test being OK, shows that the caliper/s are the problem, NOT the prop. valve.
'Mechanical slack' is an excellent phrase about your problem.
Reputable?

**gttwinturbo** · 16-12-2008, 08:26 AM

thank you all for replying. the brake rotor is the same thickness and the caliper piston seals are fine. The only other thing that I can think of is that the pin from the brake booster is not meeting the master cylinder properly and that is what is causing the "mechanical slack" feeling: when you depress the pedal you get the normal freeplay on top then it sinks to another level without pressure then there is braking pressure and of course it only gets worse when the engine is running as the booster makes it even easier to depress the pedal.

Could it be the pin from the brake booster to the master cylinder? when I reinstalled the master cylinder it just slipped right on and the pin was barely sticking out from the booster. Does this pin need to be adjusted at all and if so how is it done?. . . .damn how much bad luck can I have with this thing. . .

**allencr** · 17-12-2008, 04:14 AM

Originally Posted by gttwinturbo

Could it be the pin from the brake booster to the master cylinder?

???
You said that it was OK from between your foot on the pedal and the clamped off calipers, so isn't that pin in between, thus OK?

**oldcorollas** · 17-12-2008, 04:39 AM

have you accidentally dislodged a rattle clip, so a pad is pushed back from the disc?

you need to get wheels off and check the pads are firmaly up against the discs.. and get someone to push pedal while you try to move caliper etc...

something is not right with the calipers it seems... check all bolts done up, caliper sitting correctly, pads sitting correctly etc etc...
discs sitting correctly...

are the discs and caliper mounts the same offset betwen the two sets? or the struts are a little different?

pics?

**allencr** · 17-12-2008, 02:56 PM

Originally Posted by gttwinturbo

brake shop they clamped the two front brake hoses and the pedal was as it should be this lead them to tell me that my proportioning valve may be stuffed

'Reputable', that's BS!
Yes, they did show you exactly where the problem is but then decided to let you chase your tail by misleading you about the prop. valve.

Just like above reply, look at the caliper when the pedal is pushed and everything that is moving that shouldn't be or moving more then it should is the problem. Mechanical slack!

**Joshstix** · 17-12-2008, 03:14 PM

A good brake shop should have pressure gauges they can screw into the bleed nipple holes on the calipers to see exactly what pressure is getting to the calipers and with that info shold be able to figure ot where the issue is.

I have to say that by most standards of elimination style troubleshooting, removing one component and having the issue cease would tend to indicate that the removed component was the source of the failure though. Being pointed towards something that was present in both the successful and failed tests seems illogical.