We have a 1966 Plymouth belvedere that we race in the 24 Hours of LeMons series that has a strange phantom brake issue. If you are driving about 5 mph and turn left,the brake pedal will go to the floor if you press on it and the car keeps going. We tried a new master cylinder and that did not help. It has been converted to front disc with a correct disc master cylinder and prop. valve. The brakes work fine otherwise. Any ideas????

That is a strange one. I'd start by turning the wheel left the same amt & inspecting the flexible brake hoses on both sides. It does that EVERY time (turning left @ 5mph)? Hopefully so as that will make it easier to spot.

It does not seem to be losing any fluid either.

I'm sure you've inspected everything from front to rear and with the car stopped can you get it to act up? and power or manual? I keep thinking M/C/booster related esp since both halves(front/rear) are going out.

If it was a PB booster the pedal would be rock hard.Had a Blazer that did that.The bolt on the hose was touching when at full turn.Pedal would go down.Rocky