Please Confirm Diagnosis - Brake Booster

[Bengoshi2000]

So Thursday I was driving back to work from lunch and the braking action started to get hard. I know, that's what she said...
Yesterday I could get 'er to stop, but it was clear there was zero power assist. It was like braking on my old '66 Mustang. No problems with stopping other than the gargantuan effort required. Pedal was hard and firm, no softness. <cough>

So this morning I brought out the mityvac:

Note: My hose from the vacuum pump to the booster was replaced about 18 months ago. All vac lines and connectors were replaced 10 months ago. The check valve between the pump and the booster was working properly (i.e. I could blow air into the small orifice, but couldn't suck air out of it).

With the engine off and the MV connected directly to the vacuum pump nipple, I could pump it down to 24hg. Connected directly to the booster, 4hg.

Engine running. Directly connected to vacuum pump, 27hg. Jiggling the nipple would occasionally cause a brief drop in hg, but never more than 2 or 3. Turn the engine off and it held vac for 20 minutes. Jiggled nipple and couldn't get it to affect the vacuum.

Engine running and MV connected to the small orifice coming out of the vac pump to booster check valve, 27hg (but it took 5-10 seconds to get there). Pump the brake pedal and the vac would drop to 3 or 4.

I did check the nipple on the booster and it's remarkably loose. Tried to hold it tight to see if I could get vacuum... no dice.

Seems like the booster won't hold any vacuum. Do the minds gathered here concur? Should I check anything else?

 

[Bengoshi2000]

Anyone? Bueller?

 

[Powder Hound]

It sounds like a good diagnosis to me. Usually the problem is the supply tube to the booster, but you have eliminated that and done a good job of testing to isolate the actual problem.

Now, to replace the booster. A problem with which I have no experience and can provide no practical help. Sorry.

Good luck on the task!

PH

 

[Bengoshi2000]

I ordered the booster, mc and hard line seal from idparts (and the pedal tool from amazon). I'm hoping I might be able to get away with just the hard line seal. Not holding much hope.

As a follow up question, will any DOT4 brake fluid be fine for flushing/bleeding the brakes (post install)?

 

[UhOh]

Yes, any DOT4 fluid.

 

[Bengoshi2000]

Posting a follow-up for future searches or anyone who cares.

Booster was, indeed, the problem. I ordered the booster from iDparts and the special brake pedal release tool from Amazon. I had the old booster out and the new one in place in an hour. Truly easier than replacing any bulb in the headlight housing.

Then disaster...

I dropped the (2 o'clock position) booster mounting nut trying to thread it on. It slipped down behind the bracket that holds the throttle pedal. After nearly two hours of trying to fish around with a flashlight and a magnet, I had to unbolt the entire brake pedal / throttle pedal assembly from the firewall to shake that f*ker loose.

Let this serve as a warning... when threading the 4 booster mounting nuts on, be very careful you don't drop them. Otherwise, you will be sad.

Bled the brakes out with a Motive power bleeder and we are back on the road.

 

[AnotherPerson]

Your lucky the booster went that easily. Ours was a nightmare of a fight. Did you get the booster that works with the older master cyl you had or the new style setup that is both? Wondering how pedal feel is compared because I found with the new larger master cyl piston mine doesn't have nowhere near the travel of the old one


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[Bengoshi2000]

I put in a new booster/mc combo... already assembled. I can't tell much difference between the old setup and the new. If anything, it takes less pedal effort than before the booster failed. Pedal travel feels the same.