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.