My 93 240 has started to surge at idle but only when I press on the brake pedal. Am wondering if my vacuum booster is bad and leaking air into the intake (?)

Car at Idle in D or in Park, idles fine and smooth until I press my foot on the brake pedal. Soon after (most of the time) the idle starts to fall off and then surge back, fall off and surge back, etc. Rate is a bit slower than a turn signal. on the low end the idle gets so low the idiot light flicker. Often it wil stall.

I pulled the check valve from the booster to manifold line and it seems to be allowing flow in only one direction. I plugged the booster end of the check valve tight, left other end connected to intake and engine runs fine with no surging even after I press the brake.

Does this mean my booster is leaking vacuum? Is there a fix besides R/R of the booster? What does something like that cost?

thanks.

Don in NE FL>