Good old "two pump brake" problem.

Trondelond

Trondelond

Active Member
Ok, so I've got it. I had this problem with the OE brakes, and I still do with the APs. It is a little more pronounced now, as I guess there is a bigger area of pistons to cover. But to be honest, it's quite an unattractive feature in brakes.

So far I've noticed that some have bigger brakes, and have no problems, some have bigger brakes and problems which they have solved, and some have problems they've never figured out. Or they just couldn't be bothered with posting the solution on here.

I've bled the brakes twice, but I'll have to admit that I've done it in the wrong order according to the manual (which I haven't read until now). I bled RR, LR, RF, LF. And it's supposed to be LR, RF, RR, LF. Don't know how much difference it'll make?

Funnily, the manual states in it's fault-checking procedure :
"SYMPTOM : Unexpected pedal action -> Vehicle with ABS may have a tendency for large pedal strokes."

As far as I know, my wheel bearings are ok. I'll do the procedure to check if there's a problem with the servo or master cylinder, but they should be ok.

I don't know if I really buy the master cylinder size-theory; if this were true, I'd think everyone would have problems with their big brake kits. But - if each individual's point of reference for "pedal travel" differs greatly, it _could_ be something to consider. But I doubt it.


What I'm trying to figure out is who have had these problems, and solved them?
 
T

Tim

New Member
The pedal on my car is quite long but comes good, no matter how much I bleed it as soon as i get real heat into the brakes it goes a bit long. I put this down to the fact that i'm running a set of shitty wilwood calipers.

Aren't you on AP's, they should be excellent
 
Trondelond

Trondelond

Active Member
Yes, I am on AP's. I doubt the brand matters _that_ much, to be honest. It'd think it's still a matter of pushing a piston (or four) towards a brake pad. I just don't understand the physics involved when you have to press twice. I'm guessing that there's a build-up of pressure during the first pump, and that the second pump then applies pressure to the fluid-filled brake system.

But why - oh why - does the pressure disappear after a short while?
 
T

Tim

New Member
Air is getting in some how, it's the only answer.

FYI: Wilwood calipers are renowned for flexing
 
Trondelond

Trondelond

Active Member
Tim said:
Air is getting in some how, it's the only answer.

FYI: Wilwood calipers are renowned for flexing
Click to expand...
I stand corrected, the brand matters! :D But the caliper flexing shouldn't affect the initial pressure the fluid has?

punkcav said:
change the master cylinder, the rubbers in it might be bad
Click to expand...
Is there a way to check if they're bad before I rip it to pieces? As I've seen threads where people have replaced the whole thing, but still have this problem.
 
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gtirjoey

gtirjoey

Member
could be a master cylinder?
worth jacking the car up and checking for play in the wheel bearings aswell
 
Fast Guy

Fast Guy

Moderators
Staff member
Warped discs?

If it's doing it when stationary, get someone else to pump the brakes while you have a look and see if the pistons are moving back in or something.
 
GTIR-LOZ

GTIR-LOZ

New Member
i had this problem after turning corners was minute play in the wheel bearing on the o/s/f you really had to push and pull very hard to get movement, dont use cheap bearings from motor factors, use nissan they are alot more expensive but its not worth the hassle
 
P

punkcav

New Member
look for fluid leaking inside the car from the master cylinder, or look for a leak in the brake system air in getting in the system
 
stumo

stumo

Active Member
Trond, you've either got a bad master cyl or the pistons are too big for the size of master cyl.


what's the piston sizes?

Work out the area of the pistons on one side of the caliper.

I think the piston in the std caliper is 50mm diameter, so the area is

A=Pi r2
A=3.142 x (25x25)
A=1964mm2

you can usually get away with 10-15% more area before you can run into problems....long pedal, needing two pumps etc etc

So if your area is more than 2260mm2 then you're gonna have problems......

If you look at Willwood, Alcon, or AP calipers etc you can usually see that they're available with several different sized pistons
 
red reading

red reading

Active Member
stumo said:
Trond, you've either got a bad master cyl or the pistons are too big for the size of master cyl.


what's the piston sizes?

Work out the area of the pistons on one side of the caliper.

I think the piston in the std caliper is 50mm diameter, so the area is

A=Pi r2
A=3.142 x (25x25)
A=1964mm2

you can usually get away with 10-15% more area before you can run into problems....long pedal, needing two pumps etc etc

So if your area is more than 2260mm2 then you're gonna have problems......

If you look at Willwood, Alcon, or AP calipers etc you can usually see that they're available with several different sized pistons
Click to expand...

OOOOOoooooooooo you clever bastard :lol::lol::lol:
 
DanDud

DanDud

Member
I use to have to pump the pedal twice before getting and positive feel and effective braking, that was with the 310mm wilwoods, changed them to the K-sports = problem solved.
I know that doesnt help you solve your problem, but it just proves that wilwoods are shit!
 
stumo

stumo

Active Member
DanDud said:
I use to have to pump the pedal twice before getting and positive feel and effective braking, that was with the 310mm wilwoods, changed them to the K-sports = problem solved.
I know that doesnt help you solve your problem, but it just proves that wilwoods are shit!
Click to expand...

no, it prooves that Rally Sport chose the wrong size pistons in the calipers! :doh:
 
M

Makaveli_se7en

Member
Bigger master cylinder = problem solved. If you had the problem with standard brakes it sounds like the master cylinder have seen better days. Now it has to work harder with the AP's the problem has been exagerated. So you might as well upgrade it with a bigger one. Master cylinders seem to be the forgotten element in an uprated brake system. If I upgrade my brakes beyond the current wilwood 4 pots I will definately spec the master cylinder in line with the piston sizes.
 
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