Carrera brake booster diagnostics

[cwitt]

Hi all,

My '85 Carrera pretty much feels like it has manual brakes: Firm pedal, limited travel, and it takes some serious leg to get it to stop.

I'm suspecting something in the brake booster, so I started testing a few things. Here's what I've found...

1. I feel very little movement when I applying pressure to the pedal with the engine off and then start the engine.
2. At the hard vacuum line that exits the tub near the booster, the engine pulls 16-19 inches of vacuum at idle, with just the occasional bounce of the pressure gauge within that range. This seems good.
3. The little elbow hose between the hard vacuum line and the check valve is nice and pliable, in good shape. This seems good.
4. If I pull 20 inches of vacuum with the check valve in place, the pressure holds. This does take a surprising number of pumps on the MityVac, but once it gets there, it stays. This seems questionable. I'm still curious -- is this is really testing booster, or is it just testing the pressure between the vacuum pump and the check valve? Do I need to remove the check valve and test the booster alone?
5. If I push 20 inches of positive pressure against the check valve, it holds. This seems good.
6. Just for kicks, I capped off the hard vacuum line, left the booster disconnected, and went for a drive. The braking was worse, but not substantially. I just needed to put even more leg into the pedal to stop, and the pedal felt only slightly firmer.

Any thoughts on my questions in item 4 above?

I'm thinking I have a faulty booster. Which begs the question: Does anybody rebuild these, or must I purchase a new one?

Thanks!

[yelcab1]

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[uwanna]

After not driving my '80SC for almost 2 yrs, I took it out for a ride and had the precise braking problem you have! Bled brakes, checked for stuck calipers, etc, etc! In the end I put in a really nice booster and MC unit that I scored off Ebay, and now it brakes like new again. I had done most, but not all of the vacuum tests you did and didn't find anything that appeared to be wrong! Had me baffled, but the replacement booster was the fix.
You might check Ebay, as a lot of the "dismantlers" have good used booster/MC units available from time to time. Got mine for less than $100.
Good luck!

[cwitt]

So, I finally headed uwanna's advice and picked up a known good booster/MC. Testing it off the car, it behaves just like my current booster.

The fact that uwanna's replacement booster seemed to test the same as his original one but still resolved his issue gives me hope. But I'm also confused about what the issue could be, and am a bit hesitant to go through the work of swapping them out.

I'm curious - is it possible that my car pulls a good 16-19" of vacuum at idle at the booster nipple, but has a leak that only comes into play when the booster actually needs vacuum?

I my mind, vacuum is vacuum is vacuum, such that any leaks in my system would prevent it from pulling proper vacuum on the Mityvac.

BTW, I inspected the vacuum line that runs from the left rear of the engine, down around the rear t-bar tube and into the tub. It looks great - no signs of dryness or cracking.

Thanks,

[cgarr]

You did pop the line off at the booster after running it? it should still have a vac for a good amount of time. It should operate just fine at around 15 so if your pulling that much and its holding it must be something in the booster but other than a leak which you can test after the engine is off I don't know what it could be.

Does the car pull when your brake? collapsed rubber brake line maybe if it does?

If you hold the brake on then start the car you should feel the pedal go down a bit.

[jlynch1960]

Try comparing the vacuum at the booster vs the vacuum at the T joint immediately behind the cruise control servo in the engine compartment. I didn't have a gauge to test for actual numbers, but what I was drawing in the bonnet was a fraction of what I was drawing at that T connection. When I hooked it back up I must have cleared an obstruction in the vacuum hose between the T and the center well line - instant vacuum assisted braking (after I'd already sourced the booster through ebay). Difference was incredible. Net, net - check that section of hose in the engine compartment first - it seems to be a frequent failure point in the system.

Good luck.

[jlcinga]

cwitt, did you get it fixed? I have similar problems.