89 S4...both front breaks are not fully releasing causing break fluid to boil (assuming) and no pedal...this is intermittent after heavy break use in town ...thoughts of gentleman...?

 

Flexible hoses could have collapsed/disintegrated, blocking fluid from returning from caliper.

 

i had it 2 times on different cars... one i could indeed solve by pushing a welding rod through the flexibles and so cleaning them , the other needed a new master cilinder.

 

First, I suspect you are posting about a 'brake' problem, and not a break in your 928. IF SO, then a possibe cause and resolution follows:

If the brake fluid is not changed on a regular basis (3 years max recommended), the brake pistons can rust and nearly bond to the bore in the caliper (there are 4 pistons and bores in each front caliper). I've seen this on a couple of the 928 S4's I've owned.

My cure has been to remove the calipers, exercise the pistons VERY CAREFULLY using compressed air (with a block of wood to be sure they don't 'fly out' - THIS can be very dangerous) when they finally move, then a method to pry them back in, then repeat the procedure a few times. Eventually I'll remove the pistons and clean them thoroughly, then rebuild the caliper/piston/seals etc (might even need new pistons). Not a simple or quick, or cheap approach, but I end up with calipers that function perfectly.

Gary

 

I have heard that this can be a master cylinder issue, but when it happened on my '88 it was the pistons in the calipers sticking, almost how Gary describes. My case was a little different because on the '88 there in no dust boot on the piston and its very easy for the piston to become pitted or corroded on the outside edge and start to stick. Gary's solution of just cleaning and working the pistons might have worked, but I actually replaced the pistons (they can/could be purchased with seals). If the pistons are still available the prices will have likely changed, but I remember them as not being cheap, but not crazy expensive either. And you may only really need a few.

 

I was a witness to the brake failure when Drew drove up my driveway only to discover he had no brakes. What I noticed straightaway was the front wheels were discolored and very hot.

This was on both sides. Wouldn't that point back to the master cylinder?

 

At this point in time, stuck pistons in the late calipers has become very common.
To eliminate the master cylinder/brake booster/flexible hose to caliper, once the wheels are difficult to spin, open a front bleeder...any one of them, either side.
If that wheel then turns freely, you have a problem in the hydraulic system.....start with new flexible brake hoses.
If the wheel still doesn't turn....time for some caliper repair kits.