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1994 Starlet P8 12v cat 75hp - Brake pressure valve for rear drums broken

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On my 1994 Starlet P8 12v cat 75hp, the pressure reduction valve for the rear drums seem to be broken. I have established the problem to be the valve, and not any piping, hoses or wheel cylinder

The valve has 3 ports, 1in from main cylinder, 1 out to either side wheel. 2 pipes run back from the valve which is mounted on the bulkhead / firewall. 

1 of the outlets delivers very little pressure (allmost no braking), the other seem to deliver full pressure, not reduced as it should (brake locking).  I have plugged the defective port and put both rear wheels on the working output. Now they both brake well but lock up probably due to full pressure passing though the defective reduction valve. 

A new valve costs around 500 punds at the local Toyota dealer, and that is the only place ive managed to find the original part. 

Ive bought a generic adjustable valve from ebay, which i guess will fix the issue if i manage to adjust it right. But i wonder if that will make any changes to leak redundancy / 2 circuit braking. Both front wheels are on the same piping, from 1 port on main cyl. The rear are the same, from the other port on main cyl, but via the reduction / proportioning valve. I dont know if this valve has any functionality to reduce the effect of a leak on one of the rear pipes/ wheel cylinders, but it does not seem to be significant anyway? (a leak before the valve will cripple both rear brakes anyway, and a leak on the front will cripple both front brakes)

And second, will i have trouble woth MOT due to a quite obviously not original valve? 


All the best and thanks for all help, 



the part:
(ebay:  Adjustable Brake Proportioning Bias Valve Knob Style Suit Disc Drum 1/8 NPT BL T)


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