Brake Bias Issue

I recently upgraded my braking system to the late Gen3 brakes and discs, braided lines, non-ABS late model booster and brand new non-ABS 1” hilux 4 runner master cylinder. The brakes were formerly ABS and I have retained the ABS bias valve.
Braking is fine in everyday driving but at the track I have this issue under heavy braking where the pedal is very hard with little braking power initially, followed by a normal feel in pedal travel in the pedal and heavy braking power. In this situation the front tyres lock up easily.
 
During installation the whole system was flushed and new fluid used and the system bled well. I have since checked the brake calipers and they seem to operate and slide as they should.
 
Do any brake experts have any opinions on what the problem might be? I think I might have an issue with the circuits/bias valve. A few things to note:

1. I have retained the ABS bias valve. This has 4 ports, 2 in and 2 out. 1 of the outs splits into two for the front brakes. I have seen factory non-ABS bias valves where there are 5 ports, separate ones for each side on the front.
2. The hilux master cylinder has ‘R’ marked for the primary piston and ‘F’ for the secondary piston. I thought this should be the other way round, although I guess it doesn’t really matter as long as the brake circuits are set up as they should.
3. I have attached a photo of the master cylinder, bias valve and circuits. Im not sure if I have this right.

No I didnt know this, the car is a 1990. So a quick fix would be to get one of these check valves or in the meantime revert back to the old booster and master cylinder (track day in 2 weeks). That way I'll know for sure that this is the issue.

As I understand it, if you don't have the check valve the booster would probably be seeing manifold pressure. Under heavy throttle you'll have no vacuum, so initially the brakes would be unboosted and very stiff. Presumably you'd be off throttle and the engine will start to pull a vacuum, which will increase the braking force and return the pedal feel back to normal. The locking fronts is probably just too much brake force as the booster returns.
Reddtarga is probably on the money. Definitely the first thing you should try.

The manifold vacuum goes up and down all the time depending on throttle position and engine revs.
 
So it is very important to have a one way check valve to hold the maximum vacuum in the booster reservoir, otherwise the booster vacuum will also go up and down along with the manifold vacuum that could lead to a dangerous situation with increased stopping distances in a panic stop.
 
I don't know why Toyota changed the location of the check valve, but they must have had a good reason, and no surprise that the genuine Toyota valve is expensive, also apparently needs a special bit of pipe to mount it properly.
So if you were to fit a cheap in hose aftermarket valve, the best spot be to mount it might be back near the intake manifold the same as the later models. 
Anyone on here have an opinion on why Toyota changed the check valve setup on later models?
 
BTW, considering the number of SW20 owners that have gone to BBK's and may have also changed their booster and M/C to later model ones, I wonder just how many cars are running around without any vacuum check valve fitted.
If there are any, I hope none of them are ever driving behind me lol.
 

Reddtarga
BTW, considering the number of SW20 owners that have gone to BBK's and may have also changed their booster and M/C to later model ones, I wonder just how many cars are running around without any vacuum check valve fitted.
If there are any, I hope none of them are ever driving behind me lol.

Thats a good point, hopefully a few of those people read this post!
 
Any view on this - Can someone confirm the configuration on the hilux master cylinder is correct i.e. the primary piston is for the rear circuit and the secondary piston is for the front circuit?