Stock 2nd Gen 4 Wheel Disc Brake

[TATurbo]

Hello TAC,
 I have an '81 TA with a relatively new stock 4-Wheel Disc brake package.
I noticed something that just doesn't feel right over this past season.  I'm hoping someone here with actual brake knowledge might be able to provide some insight into the potential cause...

The brake pedal feel is normally pretty soft.  By 'normally' I mean they don't feel bad, or too soft, or way different than I remember them feeling back in the day.   But, on a few occasions this past season the pedal feel has changed to significantly stiffer. Requiring less travel to bite and less pressure to clamp down.  A feeling I would describe as an improvement over how they normally feel.

Then...boom...they go back to 'normal'. 

Anybody have a guess as to what might be going on here?

I'm working on a list of things to tweak over the winter and I think I'd like to add this to the list.  But, I have no clue where to even start looking.  Ideally I'd like to make a change to get them to stay in that 'better' feeling state, but at a minimum I'd think they should be consistent...Right?

Any insight would be appreciated.

Thanks TAC!

[JJ 109]

Sounding like a vacuum and or booster issue.

I’d start by checking for vacuum leaks and if not consider changing the booster

They do go bad and if original, not much life left. They are cheap and you can replace without opening up the hydraulic system.

[ryeguy2006a]

In addition to that, what MC are you running? I bought one from a parts store when I had my 76, and it didn't last very long.

[81Blackbird]

Agree...Check the "check valve" first.  That is the easies.  If that checks out okay, move on to the M/C.  If you have a vacuum pump, route the plumbing so that a vacuum gauge is in the line alone with a clamp to pinch the line.  I used an old check valve without the small diaphragm in it, placed the vacuum gauge between that and the vacuum pump, or manifold.  Draw down the vacuum and pinch the line.  IMO, the gauge should hold vacuum for a few days.  It's supposed to be a sealed system.  Booster rebuilders have a check but it's something like loosing 1 in-hg / 30 seconds or something, which is a passing score.   I replaced mine and the first one held vacuum for 4 days.  That one had to be replaced after 2 month and I have not been able to fine one that holds vacuum that well.  The one I have now will loose 3-4 in-hg in two hours.

What vacuum is the engine pulling?  4WD needs 18 in-hg to be effective.

[TATurbo]

Thanks for the input everyone.
The Master Cylinder and Booster were purchased together from NPD (PN C-4901-211). I don't recall what brand name they were, but the comparable PN today shows the brand as 'The Right Stuff'...Never heard of 'em.
 They are about 5 years old now with about 10,000 Miles on 'em.

I'll add the suggested checks to my winter 'to-do' list.

I appreciate the info.

[NOT A TA]

If you use the parking brake you might be causing the change you feel. If the park brake does not fully retract then the rear caliper pistons don't have to travel as far which shortens pedal travel and changes the feel of the brakes. Simple test is to try brakes then put a couple clicks or so on the parking brake and test brakes again. If it causes the changes you noticed intermittently previously you may have found the problem and just need to adjust the rear calipers or park brake. See the instructions on the Right Stuff website to adjust if necessary.

[71455formula]

Could have a stubborn air pocket in the master cylinder that comes and goes.

[langss]

If you use the parking brake you might be causing the change you feel. If the park brake does not fully retract then the rear caliper pistons don't have to travel as far which shortens pedal travel and changes the feel of the brakes. Simple test is to try brakes then put a couple clicks or so on the parking brake and test brakes again. If it causes the changes you noticed intermittently previously you may have found the problem and just need to adjust the rear calipers or park brake. See the instructions on the Right Stuff website to adjust if necessary.

Third Gens with rear disc brakes do the same thing, the difference is the "Hand Brake" application as opposed to stepping on a pedal. Because the rear calipers would not always "Ratchet Out" with wear like they were supposed to, in heavy traffic I often would pull up on the hand brake to get a little more brake from the rear if you will, since the stop and go on LA's Freeways can really heat up the fronts in a hurry.