308 GTB brake booster question

On a first (very short) drive after replacing the master cylinder, vacuum hose, and check valve, I discovered the idle rises/stumbles upon brake activation. So clearly a vacuum leak. With the brake pedal released, idle is normal (just under 1000rpm) and there is no apparent leak. Also, there is no indication of a fluid leak, as there is no decrease in reservoir level or smoke from the exhaust from fluid getting puled into the booster. So it seems to me it must be the seal around the push rod, no? This came out when I changed MCs, but it looked fine so I put in pack in. What else should I be looking for?

 

Not at the moment. My last Mityvac died. What's weird is how pressing the brake pedal causes the issue. No symptoms with my foot off the pedal.

 

Yes. The symptoms I'm having would not occur if the check valve was reversed.

 

Check valve is right next to the inlet manifold.

 

Perhaps. Checking my original work, I discovered very slightly loose clamps on both sides of the check valve. Hose is lightly corrugated PTFE with cloth over braid, which can compromise sealing. I had installed a short length of shrink tube at the ends, wrapped into the hose interior to help with the seal. Not sure how this could cause the symptoms I experienced as I would expect a high idle all the time with a leak at those points, but a quick start up and pedal check in the garage showed no tendency for the idle to change. Will verify with a drive later today.

 

Some additional information: Still having issues with braking having an occasional impact on idle speed. Also a slight and variable wonky feeling to the pedal effort at low pedal pressure. So having re-bled the brakes, removed the rear circuit pressure compensator to eliminate that variable, and checked the integrity of the new vacuum hose, I dove into the MC/booster assembly.

When I replaced my master cylinder, I measured the difference between the old and new MC pushrod relative to the respective MC housings in order to verify if the booster pushrod needed adjustment, which it did. So I removed the booster pushrod and shortened it the required .025".

I now think removing that pushrod was an error, as now it is loose in the booster, as I discovered when I removed the MC again. As I recall the booster pushrod required a tug with pliers to originally remove it. I thought I had it inserted in the correct location and assumed it would seat itself with the first pump of the pedal.
Try as I might, I can't get it positively seated. Inspection with a boroscope does not show any obvious damage, but something is obviously amiss. So tomorrow I will be working on removing the booster.

Anyone faced this in the past? Any advice?

 

Not what I was experiencing. What I noticed is a slightly different pedal height from one light brake application to the next. No variation in height once the pedal was pressed and held.