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?

 

Do you have a Mityvac tool you could use to pull a vacuum on the long hose to the booster to see if it holds vacuum?


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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.

 

Check valve is correctly oriented and working right?


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

 

That has happened to me on another car. The booster seal where the hose connects got knocked a little loose so whenever I would press the pedal the idle would change. I'm not sure on the 308 but my other car had a right-angle plastic connector that the hose connected to and that right-angle connector is held in the booster with a rubber seal. If you can sit there at idle and pump the brakes and the idle runs up, then there is probably a leak around the booster where the hose runs in there, especially since it is in the proximity of where you were working.

 

Hi, did you put the check valve at the front, near the booster, or close to the carb? I ask because I ran into problems with the valve near the brake booster (as shown in the parts diagram) since this results in the long tube acting as a reservoir that has to continuously get emptied, hence idle and carb issues.

 

Check valve is right next to the inlet manifold.

 

Did you solve it?


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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.

 

The way a booster works is vacuum is applied to both sides of the diaphragm. When you depress the brake pedal it simultaneously seals the port between the front and rear and opens the rear side to atmosphere. The symptom you described would indicate either a ruptured diaphragm or the inter-chamber seal was leaking. The purpose of the check valve is to maintain vacuum in the booster when the engine is off so that if you, for instance, were to step on the clutch to start the engine and the car rolled forward or backward you'd still have brake boost to stop it. So I doubt leaks at the check valve would cause your symptom. Perhaps the inter-chamber seal was leaking but somehow resealed itself? To test it you would apply vacuum to the booster with something like a MityVac then step on the brake pedal and see if it holds.

 

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?

 

Off topic, but I’m curious what it was feeling like at low pedal pressures. I don’t have the idle speed problem but it was interesting yesterday when I came to stops and with my foot just resting on the brake enough to keep me from rolling, the pedal would rise up with a couple bumps and then settle back down. I thought it was curious but t didn’t think anything of it until I just read your post. I haven’t really driven this car much yet so that was really the first time I stretched its legs.


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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.

 

Maybe time for a brake fluid flush?

 

For sure.


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