Bias problems, locked rears and spun car twice on track

[thebutterson]

Hello everyone, I'm looking for some help troubleshooting/understanding this issue I had yesterday. It was my first time out with this specific E30, but I've previously tracked another E30 although it was set up much differently. Twice the car spun on me while braking in a straight line under moderate to hard pressure. Both times I progressively applied pressure and once I got to a certain point the car snapped on me and I spun. Fortunately I spun almost 360 degrees both time in a relatively straight line and no damage was done other than some flat spots in the tires that I can feel at speed. There were a few other times I as able to feel the back getting unstable and save it and another small lock event that I was able to let off the brakes and countersteer.


Brake setup:
325i stock calipers F/R
Vented/Slotted rotors Front
Plain rotors Rear
PFC 08 pads F/R
Torque T700 fluid
25mm MC
No ABS (early 318i didn't have it)
OEM proportioning valve in place

Car:
1985 318i no ABS and originally had rear drums
Honda K24A2 engine
Old but still sticky used Direzza ZII 225/45/16 tires on 16x7 wheels
325i/e H&R sports and bilstein sports
Stock 318i front sway bar, no rear sway bar (oem setup)

I don't really understand why the rears locked up as I've seen posts saying to delete the OEM pressure limiting valve to get MORE rear bias when going to an MC with the same piston sizes. I still have mine in place although I'm starting to suspect it has failed. There were some times when I was still on the original 318i MC and drums that I locked up the drums with only moderate braking on the street.

Ideas:
-Change the pad setup so the fronts have more bite than rears
-Remove stock pressure limiting valve and replace with aftermarket adjustable valve to further limit rear pressure
-The OEM pressure limiting valve seems to be NLA, is there a different option? Something equivalent from another car?
-Relatively soft suspension allowing nose to dive or rear to raise too much causing toe change that would lead to instability

[hoveringuy]

The Wilwood proportioning valve is a drop-in fit. Double-check that this is the right one, Wilwood Engineering 260-12627

Is the MC 25mm front AND rear or stepped to a smaller bore for the rear?

Any restrictions or bends in the front lines?

Your setup is identical to what I used to run without issue... (except I still had the ABS)

[thebutterson]

I ended up installing the Wilwood proportioning valve and that did the trick. After inspecting all of my lines and exercising all of the brake calipers to make sure the pistons easily extended I figured there was nothing wrong with the brakes other than the stock pressure limiting valve possibly being bad. I went back to the track and started with the valve almost in the most forward biased position and slowly turned it in to add more rear bias. After a few turns I went from locking up the fronts relatively easily to being able to brake pretty hard with no lock up. I probably haven't achieved the optimal spot, but it is certainly much better and much safer than it used to be. Locking up the fronts was significantly less scary than locking up the rears.

[kronus]

If the stock valve was from the original drum brake setup, it is a residual pressure valve, not a limiting one. That might have been your problem if the car got a rear disc conversion.

https://help.summitracing.com/app/answers/detail/a_id/5297/~/what-is-a-residual-valve%3F

[88Sedan]

Just found this thread. The comment above is absolutely correct. Drums require more brake pressure to work correctly, so you swapping to discs was delivering way too much braking power to the rear.

[thebutterson]

If the stock valve was from the original drum brake setup, it is a residual pressure valve, not a limiting one. That might have been your problem if the car got a rear disc conversion.

In the process of doing the rear disc conversion I looked on realoem to see what would else I'd need to change other than the MC when going to discs. Real oem lists the car as having part number 34331152494 and calls it a Brake Pressure Regulator and lists it for basically all models.

What came out of my car looks just like this:
BMW Brake Pressure Regulator - E30 - 34331152494 (bimmerworld.com)

Which bimmerworld also lists for all models. I was able to confirm the same 5 down arrow 25 markings as the google images show, although on mine they're stamped into the wrench flats, not on the body.

[Digitalwave]

The 25mm MC is not a good "upgrade" for 325i brakes, in my opinion. The stock 325i master cylinder had staggered bores -- 22mm front and 17mm rear. Going to the 25.4mm front and rear bores of the E32 MC drastically changes the brake bias rearwards. I don't know if the 318i prop valve is appropriate for the 325i brakes. It may or may not be, but that would be fairly easy to swap out. Perhaps try going back to a 325i stock master cylinder rather than fiddling with an adjustable bias valve.