Brake pedal slow to return

Hi everyone. I am trying to trouble shoot an issue that I am having with a 1990 Testarossa, US version, but likely isn’t unique to just that version. The issue, as the title eludes to, is my brake pedal is exceedingly slow to return to the up position at certain times. Of note, I recently had the calipers redone for a different issue, but I don’t think this is related. I had brake pedal return issues prior as well. Here are some specifics:
The issue is really only noticed when I press the brake pedal AND the clutch pedal at the same time. Brake pedal alone with engine on or off, and the pedal returns quickly (within a second of release). Brake pedal with the clutch pedal down and it takes multiple seconds for the brake pedal to return to the “up” position. The return time is longer with successive brake pedal pushes and with the engine running.
So things I don’t think it is—soft brake lines (I have replaced those), brake master cylinder (I had this rebuilt), brake pads or calipers (new or rebuilt). And I have bled the brakes and clutch per the work shop manual or owners manual (which really aren’t detailed at all).
Any suggestions or experience with something similar? I really appreciate the help.
Thanks,
Spike

 

I can hear some sounds from the brake booster, but I don’t think it sounds abnormal for a booster. I could be wrong, obviously. One check I have done on the booster is to press down on the brake pedal with the car off and continue pressing. As I start the engine, the brake pedal does depress slightly. I think this tests at least part of the booster as normal.

 

Hi, Steve. I did do a search and as you said, this doesn’t seem to be too common. The return spring is normal on the brake and clutch pedal.

 

Thanks, Romano. I will certainly check those. But based on when the delayed brake return was occurring, I thought it was more related to a pressure issue. That was leading me to believe it was either related to a brake booster internal problem or possibly a clutch master cylinder issue. I can’t seem to figure it out, though.

 

Quazipsycho, I seem to notice mine more in colder temps. But I did use all new fluid in the bleeding process.
Romano, I only thought the clutch master cylinder could be part of the issue since the brake pedal issue really only occurs when the clutch pedal is also depressed. I think this symptom may have more to do with the pressure on the overall fluid system instead of the actual clutch cylinder. As I’m sure you already know, both systems share fluid from the same brake fluid reservoir. And similar to your experience, I could use my foot to bring the brake pedal back to the raised position. It would return on its own, but very slowly.
I also considered the vacuum servo valve (part #5 in Romano’s supplied diagram above), but it appears to work normally. It allows some airflow one direction only.
After everyone’s comments, I guess I am leaning more toward this being a brake booster (or brake servo in some circles) problem. I am just kind of thrown by the brake pedal being normal unless I also depress the clutch pedal.

 

I’ll give that a shot and see how it goes. Thanks for the recommendation.

 

Yesterday, I took the opportunity to remove the driver’s seat and do some body contortions to clean and lubricate the pivot points that you all recommended I check. Really, they looked fine with just a little bit of grease showing on a few of the pivot points. The more I considered it, the more it made sense that an increased torsional effect of pressing the the clutch pedal could directly affect the release of the brake pedal if the grease was causing it. This could also be worsened by the colder temperatures, which was what I was experiencing.
The result is that the brake pedal returns much more consistently and in a much quicker fashion after my attempts to lubricate those points. Thanks again for everyone’s help in narrowing down the possible fault. I really appreciate it. I think I need to drive it more to ensure it doesn’t “gum up” in the future.
Spike