stumo
Active Member
true but why not just buy calipers with the right size pistons in the first place?:roll:
Good old "two pump brake" problem.
- Thread starter Trondelond
- Start date
true but why not just buy calipers with the right size pistons in the first place?:roll:
red reading
Active Member
Because that is too easy:lol:
Makaveli_se7en
Member
Depends on the size of pads you want to run, no point having silly little pistons pushing the middle of the pad is there? It also leads to the centre of pressure hunting around the pad under braking which in turn leads to noisy bastard brakes
red reading
Active Member
And Brembo,tokico,nissen,sumitomo
Trip
New Member
AP calipers: No, since the problem was there on standard calipers
Wheel Bearings: No, since the problem is also noticable when car is stationary
Leak: check fluid level
Servo: Engine off, pump pedal till hard, Keep your foot on it and switch on engine. If it goes down, its OK
Master Cylinder: Engine off, pump pedal till hard, Press pedal hard for a minute. If it does not go down, its OK.
I had a different problem on mine. Brakes were very weak. I ended up changing the Bias valve to a non ABS one, Master cylinder, and upgraded to 4pot wilwood (with 35mm pistons). I will test tomorrow or saturday.
Wheel Bearings: No, since the problem is also noticable when car is stationary
Leak: check fluid level
Servo: Engine off, pump pedal till hard, Keep your foot on it and switch on engine. If it goes down, its OK
Master Cylinder: Engine off, pump pedal till hard, Press pedal hard for a minute. If it does not go down, its OK.
I had a different problem on mine. Brakes were very weak. I ended up changing the Bias valve to a non ABS one, Master cylinder, and upgraded to 4pot wilwood (with 35mm pistons). I will test tomorrow or saturday.
Trondelond
Active Member
I checked the servo the other way stated in the manual - pressure on pedal while car running, turn the car off and see if the pedal has dropped in 30 secs.
I'll do the master cylinder like you propose! I suppose overhauling it is no harder than the clutch cylinder?
And thanks for the formula, stumo. I'll try to figure out the area!
I'll do the master cylinder like you propose! I suppose overhauling it is no harder than the clutch cylinder?
And thanks for the formula, stumo. I'll try to figure out the area!
Trondelond
Active Member
So;
OE piston is 54mm
3.142 x (27 x 27) = 3.142 x 729 = 2290.1 mm2
AP Pistons are 38.1mm
3.142 x (19.05 x 19.05) = 3.142 x 362.9 = 1140.2 mm2
But there's two pistons on each side :
1140.2 x 2 = 2280.4 mm2
So, if your theory is correct, it's the master cylinder!
watoga
Member
If the AP's have *smaller* area than the OE pistons, then surely the applied pressure should be greater?
Pressure = Force / Area
So, I think stumo's worry was that if AP's piston area was huge compared to the OE brakes, then the pressure would be substantially decreased. However, since the combined AP's area is less than OE, the problem might be coming from elsewhere.
How about the junction in the caliper where the hydraulic fluid is split between the two cylinders? Can this be checked? Is there any flexing in the hydraulic lines?
Hope you get it sorted soon,
Dave
Pressure = Force / Area
So, I think stumo's worry was that if AP's piston area was huge compared to the OE brakes, then the pressure would be substantially decreased. However, since the combined AP's area is less than OE, the problem might be coming from elsewhere.
How about the junction in the caliper where the hydraulic fluid is split between the two cylinders? Can this be checked? Is there any flexing in the hydraulic lines?
Hope you get it sorted soon,
Dave
Trondelond
Active Member
The only thing I'm concerned with is that this is a four-piston caliper, and thus it has roughly double the total area of the OE piston, but stumo's theorem (as it shall be known as henceforth) says that thou shalt only calculate the area of one side.
I don't think I can check that junction - no. I don't know if they flex, but I can check.
I don't think I can check that junction - no. I don't know if they flex, but I can check.
watoga
Member
Is the standard OE brakes single-piston calipers?!?!?!?! My God, no wonder they are so crap! LOL!
If this is the case, then you are correct:
OE total area = 2290 mm2 (one piston)
AP total area = 4560 mm2 (four pistons at 1140 mm2 each)
So this suggests the master cylinder is inadequate for your needs
Is an upgraded master cylinder with a wider bore easy to fit?
Cheers,
Dave
If this is the case, then you are correct:
OE total area = 2290 mm2 (one piston)
AP total area = 4560 mm2 (four pistons at 1140 mm2 each)
So this suggests the master cylinder is inadequate for your needs
Is an upgraded master cylinder with a wider bore easy to fit?
Cheers,
Dave
Trondelond
Active Member
Yessir, they are!
As far as I'm aware, there are no replacement cylinders. At least not that I would be able to fit on my own. I may (hopefully) be wrong.
However, other people have been using even bigger brakes without problems - old loveable sourpuss Odin comes to mind - and I don't think he had upgraded his.
And there's even a six-pot kit that works : http://www.gtiroc.com/forums/showthread.php?t=50896&highlight=k-sport+6pots
I think I'm going to have to do the master cylinder and check wheel bearings before I make any more assumptions.
As far as I'm aware, there are no replacement cylinders. At least not that I would be able to fit on my own. I may (hopefully) be wrong.
However, other people have been using even bigger brakes without problems - old loveable sourpuss Odin comes to mind - and I don't think he had upgraded his.
And there's even a six-pot kit that works : http://www.gtiroc.com/forums/showthread.php?t=50896&highlight=k-sport+6pots
I think I'm going to have to do the master cylinder and check wheel bearings before I make any more assumptions.
Trondelond
Active Member
*adding it to the checklist*
stumo
Active Member
If you're going to calculate the area for all the pistons then you have to double the area for the std caliper therefore it's easier just to consider only one side of the caliper.
Calculating the areas of the pistons at this stage has nothing to do with the pressures involved , it's to work out if the piston areas are similar or not.
If the area of the "uprated" pistons is a lot larger than the std pistons then you will get a long brake pedal as the volume of fluid moved by the master cyl isn't sufficient for the new calipers.
I now have the K-sport 6pot kit and from memory the piston area was very similar to std and i have no problems with any excess pedal movement.
Calculating the areas of the pistons at this stage has nothing to do with the pressures involved , it's to work out if the piston areas are similar or not.
If the area of the "uprated" pistons is a lot larger than the std pistons then you will get a long brake pedal as the volume of fluid moved by the master cyl isn't sufficient for the new calipers.
I now have the K-sport 6pot kit and from memory the piston area was very similar to std and i have no problems with any excess pedal movement.
Last edited:
Trondelond
Active Member
I don't understand the physicis of it, as I'm checking the total piston area of the OE caliper against half of the piston area of the AP caliper. But as I don't understand it, I'm not going to argue.
However, had to go out to the car now, and I tried pumping the pedal hard while stationary. It kept the pressure, and was firm after 30 seconds. Drove a few yards and weaved to try and provoke the bearings, and the pressure dropped. I think I may be on to something here.
However, had to go out to the car now, and I tried pumping the pedal hard while stationary. It kept the pressure, and was firm after 30 seconds. Drove a few yards and weaved to try and provoke the bearings, and the pressure dropped. I think I may be on to something here.
stumo
Active Member
Is the standard OE brakes single-piston calipers?!?!?!?! My God, no wonder they are so crap! LOL!
If this is the case, then you are correct:
OE total area = 2290 mm2 (one piston)
AP total area = 4560 mm2 (four pistons at 1140 mm2 each)
So this suggests the master cylinder is inadequate for your needs
Is an upgraded master cylinder with a wider bore easy to fit?
Cheers,
Dave
you're completely wrong, as the OE caliper is a sliding one you have to treat it as a 2 piston caliper.