Rear brakes locking up, any advice?

[my_mad_z]

Hey guys, hope you are all well. Sorry in advance for the long winded post, just trying to provide all the info I can.

I had an incident a few weekends ago where I had to brake hard in my Z and it scared the absolute sh*t out of me. I was doing 100kph on the freeway and the car in front slammed his brakes on; I jumped on the brake pedal; the rear brakes locked up instantly and sent me into a spin. I tried my best to correct it but it was a panic brake and I ended up losing control; I know it wasn't the right thing to do but I'm pretty sure I had my foot buried on the brake the whole time

I have the following brake setup:

Front brakes converted to a vented disc from a Peugot 504 with Toyota 4 pot calipers (SW12 from memory);

Rear converted to disc using the R31 Skyline discs and calipers;

Master cylinder is 7/8 (not sure if it's a 240Z or 260Z unit) with the original booster (small booster on the early 240's).

I did some reading up on what may have caused this and I came across Locky's write up on the rear disc conversion and realised I didn't remove the plastic/rubber valve out of the master cylinder from the rear line when I did the conversion; the conversion was done long before Locky's write up. I pulled the MC apart and inspected everything, all looked ok. I removed the rubber/plastic valve and in the process I noticed that the front line had the same valve so I've removed that as well. Checked all of my calipers and they are all in working order, no seized pistons, no leaks and everything moves freely. I put everything back together and bled the brakes. I checked the Haynes manual and it suggests to brake from 50kph and if the rears lock up, it's likely to be the proportioning valve. Took it for a drive and did the test and the rears locked up.

I decided to pull the proportioning valve out from the rear of the car and pulled it apart. It was full of gunk but the seals and bore appeared to be ok so I cleaned it all out and put it back in. Bled the system again and took it for a drive and did the same test and it the rears still locked up. My MC lines aren't labelled front or rear so I tried swapping the lines over to see if that would make a difference; no difference, rears still lock up.

I don't think the proportioning valves are available anymore and I'm not sure what else to try now? Anyone got any suggestions? I'm considered using a manual proportioning valve and deleting the existing valve from the rear, I can't see what else would be causing the issue? I'm not 100% sure if the proportioning valve is the issue or if something else can cause the rears to lock up? The only thing I haven't pulled apart is the brake pressure switch located in the engine bay to see if that's stuffed or not? Not sure if that would cause the rear brakes to lock up?

I've been looking at upgrading to the 15/16 MC but I had a chat with Tai and he told me about the R34 MC/booster upgrade that he did that works a treat with the Toyota/R31 caliper upgrade so I'm going down to the wreckers on Saturday to see if I can get one to try it out.

I'm open to any advice or suggestions on what to check/try.

Thanks in advance.

Marc.

Edited April 25, 2019 by my_mad_z

[Gordo]

if you have apportioning valve it typically will adjust the rear brake line. you should be able to wind the rear brakes off.

[my_mad_z]

Thanks Gordo, I'm looking at getting a manual proportional valve so I can adjust the biasing. I'll try the master cylinder and brake booster upgrade to see if that makes any difference and if not, I'll be adding the valve.

Edited April 26, 2019 by my_mad_z

[Riceburner]

Any difference in the pad compounds front to rear?

[my_mad_z]

Brake pads are the same brand front and rear however tyres are different. Fronts are Bridgestone Potenza and rears are Nitto semi slicks... I'm going to try swapping front/rear tyres to see if that makes a difference. 

Edited April 26, 2019 by my_mad_z

[my_mad_z]

Looks like the R34 master cylinder has a proportioning valve built in for the rear so I ended up gutting the proportioning valve (located in the rear of the car) and deleted the pressure switch in the engine bay as I suspect there's an issue with it. Brakes feel so much better now and the rears are no longer locking up. Just have to wire the switch of the master cylinder reservoir and its done. Can't believe the difference in braking since the MC/booster upgrade! 

[PeterAllen]

You may want to research this further but my understanding is that it is not a proportioning valve at the rear of the car. It is a device which allows more fluid to drain from the rear slave cylinders and thus ease the shoes away from the drums. It creates a tiny reservoir in the line to the rear brakes and the brakes subsequently run cooler but also take a little longer  (nano seconds?) than the fronts to start working.

If you have rear drums you should keep it, if you have rear disc you don't need it but I suspect it doesn't matter.

https://postimg.cc/yJDCG08w

Edited May 12, 2019 by PeterAllen

[my_mad_z]

Hi Peter,

Thanks for the info. I converted the rear from drum to disc many years ago; looking at the Haynes manual and other resources online, they label it as a proportioning valve so that's why I used that terminology:

 

I'm suspecting there may have been an issue with my 'Warning Light Switch' (item 8 ) because after deleting it, the pedal and brakes felt much better.