strange brake problem

Can anyone suggest a cure for my UK Audi 80, I am at my wits end. When pressing the brake pedal, the pedal was occasionally going nearly to the floor, when pumped twice it would be OK and the brake pedal pressure
would be fine for days, I replaced the master cylinder with a new one from Europarts UK and everything has been OK for a week, now the original problem has happened again, brake pedal will go to the floor and pumped twice it will have a good brake pedal and the brake pedal pressure would be OK, brake pedal pressure is good and when braking car does not veer from a straight line, can anyone diagnose this problem as to why brake pedal pressure keeps dropping from time to time? can't see any leaks, master cyl reservoir fluid level is not dropping Ps my audi 80 1.9TDI avant has got ABS discs brakes all round, 1994 model. Thanks
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

The brake circuit is simple and there seems to be nothing wrong. Keep in mind that also the clutch is getting it's fluid from the reservoir. Maybe there is something wrong?
Just my 2 cents...
Ronald
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Yes the clutch is fed from the reservoir too, nothing wrong with the clutch operation, could this fault be caused by a sticking brake calliper?, the brakes are ok when the brake pedal pressure is fine, does not pull from side to side, just a thought? I will jack up the car and check each wheel, it's annoying you want to know that you have good brakes all the time:))
Nad

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

No, all brake calipers kind of stick. On disk brakes calipers are not retracted.
My guess is -if not air in the system- also a lousy master cylinder.
Ronald
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Nadia wrote:

Your new M/C is likely junk. Especially if it was a "rebuilt" rather than "new" piece.
Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
It was a new master cylinder not a refurbished one, I will contact the supplier on monday.
Nad

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
What is your brake fluid level doing, are you loosing any, check at the wheel (slave) cylinders as well. Best of luck, John 1990 V8 Quattro
wrote:

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Hi, brake level fluid never drops in reservoir, no leaks at any calliper (pistons), today drove the car for 2 hours brake pedal firm, not spongy, car brakes very well, after 2 hours, brake pedal goes to the floor little pressure, pumped the brake pedal 2 twice got a firm brake pedal, then brake pedal went to the floor again, pumped brake pedal twice got a firm brake pedal again, brake pedal after driving another hour is firm and have good brakes no sign of pressure dropping!! Still looking for advice guys, I changed the master cylinder with a new one when this symptom first happened, I guess the original problem was not the master cylinder after all, I would have thought that problem is not a master cylinder, since the problem still occurs. checked so far.. No fluid leaks on ground. Brake reservoir level does not drop. car brakes in a straight line, tried this many times, perfect braking, Handbrake (discs) holds car perfectly, and releases with no dragging, don't think there is a seized piston (calliper) What else is there to check, brake pressure valve? Servo? when the brake pedal pressure fell I stopped, switched off the engine and pressed the pedal a couple of times and the pedal still went to the floor ABS???
Nad
<John> wrote in message

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Suspect air in the system then. Have you bled the system carefully, at master cylinder then each caliper in turn? The order is Master Cyl / RH Rear / LH Rear / RH Front / LH Front. When you are bleeding the rear cylinders you need to press the brake regulator lever also. (you'll need an assistant!)

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload
Could also be water in the (old?) brake fluid. If cold no problem, after heating up it becomes steam which can be pressed together ---> strange breaking behaviour. (I hope you understand what I mean, english is not my native tongue)

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
<%-name%>
Add image file
Upload

Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.