Hesitation in acceleration on my 92 audi 80 quatto

Sounds like a vac leak in the power brake booster.

The most obvious cause for a hard pedal is simply not enough vacuum. Check the brake power booster by pumping on the brake pedal while the car isn't running. Continue pumping until you've "bled off" the vacuum from the booster. Hold the pedal down while you start the car. The pedal should go down a bit more beneath your foot. If it doesn't, then you should examine the connection between the vacuum hose of the brake power and the engine's vacuum.
Replace the brake power booster if the connection's fine, which would fix the hard brake pedal problem.

It may be your calipers need replaced. Check them for signs of wear. Look at the metal spring where the brake pads sit.
Apply tension to the pad and see whether you hear a pop. If you do, the brake pad is moving too much on the bracket. Replace the caliper. This could help the hard brake pedal.

Booster problems are rare but they do go bad sometimes.
A bad booster generally causes the brake pedal to feel hard to push down, and a lot of pressure on the brake pedal is required to make the car stop.
Rarely, a vacuum brake booster will develop a vacuum leak, which can cause engine performance problems, hissing or whistling noise, as well as requiring increased pressure on the brake pedal to stop.

The first test:
With the vehicle parked, turned OFF, and the parking brake set, pump the brake pedal 5 to 6 times. If the pedal gets harder to push after several times, your booster is probably good.

The second test (To make sure):
After the test above, without making any changes, press and hold the brake pedal with light to moderate pressure.
Now with your foot still on the brake pedal, start the vehicle.
If the booster is good, you will feel the pedal go down when the engine starts.

Hope this helps!

sounds like there is a vacuum leak, a vacuum leak will cause your engine to run lean, that may cause your idle speed to increase, , check the vacuum line that goes from the rear of the engine to the power brake booster, it may be the brake booster as well, ether of these will cause hard braking, hope this helps.

You can only have whatever the engine vacuum is
at any time.
Is there vacuum to the booster check valve
& is the valve working,assuming you have the
old one or a new one with the new booster.
Does fluid flow out all 4 corners?
Are the Rubber Flex Hoses Collaped
Calipers Rusted & frozen?
Did you remove the master & clean out
the moisture from around the piston
That is where you start,then flush & on to
every component in the system, if they are
on the vehicle for many years
You do realize brake fluid gets changed every
2 years on all vehicles or you got big problems with
moisture in years to come

NO. the vacuum is used to make the brake pedal easy to push, power brakes. the pedal would be hard without vacuum, that is why the pedal gets hard with the key off and engine not running. the problem you are having is more of a hydraulic problem.
Have the brakes checked asap. the brake system may have a leak causing the pedal to go low. check the fluid level and if it is low or empty then there is a problem, could be a rotted brake line leaking or a wheel cylinder leaking.

A hard brake pedal is caused by a failed and leaking vacuum diaphragm in the power brake vacuum operated booster. Another reason is loss of vacuum to the brake booster, buy you should hear a loud sucking noise, also the 3/8" vacuum hose can collapse from age check that as well.

the booster is the problem, or a hole or leak in vacuum line to booster check the check valve also ! from manifold. heres the symtoms of a bad brake booster. primary symtom is a hard pedal on application, to test booster. pump brakes several times with engine off. to deplete stored vacuum. turn on engine with pushing slightly on brake pedal. you should feel the pedal fade away abit and then become firm. but not hard. if you feel nothing at pedal when engine starts, brake booster is not working! check the vacuum lines first!!

Your power brake booster is controlled ny vacuum so if it was going bad or if you had a leak in the vacuum hose that connects to your brake pedal would be hard. I hope this helps and good luck.

Inspect brake booster vacuum line from engine intake to brake booster for leaks also inspect brake booster check valve, more than likely your brake booster is causing a vacuum leak to your Intake thus leaning mixture wich increases idle rpm