Odd fitting on Master cylinder

[deathbypsi]

After I built my coilover hoops I found I couldnt use the stock master with the brake lines coming out the side facing the fender due to clearance issues. Somebody told me to get a new master off I believe a 88 STX cause the brake lines went in on the other side facing the motor.

I have the new master with the fittings on the side facing the motor solving my clearance issues but the rear port uses a different fitting and Im not sure what it is. The front port is normal and takes a 3/16 flared fitting,however the rear port has no seat for a brake line to seal against and appears to be a straight thread.It is not a NPT pipe thread.

Anyone have any idea what fitting goes into this port?

 

[adsm08]

The rear part doesn't have the female side of the cone, like a metric ISO flare does it?

 

[deathbypsi]

It appears to be a 9/16-18 thread but the bottom of the port is flat with just a small hole drilled in middle. No tapper,no flare and no seat of any kind. Image I found on Ebay shows an O-ring should have came with the master but I never got one with mine.

 

[adsm08]

Huh, never seen a brake fitting with no flare to it.

 

[canyoncritter]

residual valve

 

[Sasquatch_Ryda]

Ya its probably for a residual valve, which has just a pipe thread on it and the valve itself has the seat for the flare. I used a 94 fsuperduty master on my 91 when I swapped in the one tones. You can see the valve screwed into the rear port.

 

[deathbypsi]

Isnt the rear port for the front brakes though? Why would you have a valve going to the fronts?

 

[Sasquatch_Ryda]

The brake pressure control valve regulates the hydraulic pressure in the rear brake system. It is located between the rear brake system's inlet and outlet ports in the main valve. When the brake pedal (2455) is applied, the full brake fluid pressure passes through the brake pressure control valve to the rear brake system until the valve's split point is reached. Above its split point, the brake pressure control valve begins to reduce the hydraulic pressure to the rear brakes, creating a balanced braking condition between the front and rear wheels to minimize rear wheel lock-up during hard braking.
In case of the front brake system malfunction, the brake master cylinder fluid control valve has a bypass feature, which allows full hydraulic pressure to the rear brake system.