brake experts, please help! out of ideas...
Discussion
ok, i've got a binding front nearside brake on my e36 m3. ive replaced discs, pads, calipers, flexi hoses, had system bled and it still locks the caliper really tight when i brake on that side. other side is fine.
the car is at the garage this evening but we're out of ideas. i've got to go back and look at it fresh tomorrow morning.
so, please, pretty please... can anybody suggest any other causes? everything has been replaced with brand new OE parts.
the car is at the garage this evening but we're out of ideas. i've got to go back and look at it fresh tomorrow morning.
so, please, pretty please... can anybody suggest any other causes? everything has been replaced with brand new OE parts.
cheers for the replies guys.
sliders are greased and move fine, no issues. the carrier is fitted from the previous caliper which also bound and is now fitted with a brand new caliper and is still binding.
is there a brake proportioning valve on these cars, or could it be a master cylinder symptom possibly? i know very little about hydraulic brake systems so really just clutching at straws now.
sliders are greased and move fine, no issues. the carrier is fitted from the previous caliper which also bound and is now fitted with a brand new caliper and is still binding.
is there a brake proportioning valve on these cars, or could it be a master cylinder symptom possibly? i know very little about hydraulic brake systems so really just clutching at straws now.
Superhoop said:
Has it got ABS?
I would assume it probably has - it could be the valve in the hydraulic unit that operates that caliper sticking.
If it were the master cylinder, then you would expect the opposite side rear brake to also have an issue, as they are a crossed circuit
that's worth knowing, cheers. I would assume it probably has - it could be the valve in the hydraulic unit that operates that caliper sticking.
If it were the master cylinder, then you would expect the opposite side rear brake to also have an issue, as they are a crossed circuit
i'm showing my ignorance but the ABS pump just drives the front two wheels right? so there are split lines leaving the abs pump, one to OS and one to NS?
Not exactly - the master cylinder will have two pistons, operating a crossed circuit, so one piston of the cylinder will provide pressure for the LHF and the RHR, the other will provide pressure for the RHF and LHR.
The reason for a crossed circuit is to try and ensure the car can still brake in a relatively straight line should one of the 2 brake circuits fail - cars used to have LH and RH brake circuits, but this meant that a failure of one circuit left the car with brakes on one side of the car only meaning they would pull heavily to one side
If it has ABS, then the crossed circuit will supply pressure ABS hydraulic unit through two pipes, this will then be split by the ABS hydraulic unit to the front and rear brakes - The HU normally modulates the required difference in brake pressure between the front and rear brakes too, although some cars do still use a regulating valve for the rear axle.
ETA: to clarify, the ABS hydraulic unit will control all 4 brakes, and should have 6 lines in total - 2 in, 4 out
The reason for a crossed circuit is to try and ensure the car can still brake in a relatively straight line should one of the 2 brake circuits fail - cars used to have LH and RH brake circuits, but this meant that a failure of one circuit left the car with brakes on one side of the car only meaning they would pull heavily to one side
If it has ABS, then the crossed circuit will supply pressure ABS hydraulic unit through two pipes, this will then be split by the ABS hydraulic unit to the front and rear brakes - The HU normally modulates the required difference in brake pressure between the front and rear brakes too, although some cars do still use a regulating valve for the rear axle.
ETA: to clarify, the ABS hydraulic unit will control all 4 brakes, and should have 6 lines in total - 2 in, 4 out
Superhoop said:
Not exactly - the master cylinder will have two pistons, operating a crossed circuit, so one piston of the cylinder will provide pressure for the LHF and the RHR, the other will provide pressure for the RHF and LHR.
The reason for a crossed circuit is to try and ensure the car can still brake in a relatively straight line should one of the 2 brake circuits fail - cars used to have LH and RH brake circuits, but this meant that a failure of one circuit left the car with brakes on one side of the car only meaning they would pull heavily to one side
If it has ABS, then the crossed circuit will supply pressure ABS hydraulic unit through two pipes, this will then be split by the ABS hydraulic unit to the front and rear brakes - The HU normally modulates the required difference in brake pressure between the front and rear brakes too, although some cars do still use a regulating valve for the rear axle.
ok, now i'm MORE confused.The reason for a crossed circuit is to try and ensure the car can still brake in a relatively straight line should one of the 2 brake circuits fail - cars used to have LH and RH brake circuits, but this meant that a failure of one circuit left the car with brakes on one side of the car only meaning they would pull heavily to one side
If it has ABS, then the crossed circuit will supply pressure ABS hydraulic unit through two pipes, this will then be split by the ABS hydraulic unit to the front and rear brakes - The HU normally modulates the required difference in brake pressure between the front and rear brakes too, although some cars do still use a regulating valve for the rear axle.
so, in laymans terms if this was your car, the hydraulic system is locking just one front wheel, where would you put your money?
Well normally, I'd say caliper or brake hose, but you've done that already, so you already know it's not these.
If it was one piston in the master cylinder sticking and holding pressure, I'd expect to see an issue with the opposite rear corner as well as the front as it will be supplying pressure to NSF and OSR so in this case, I'd rule it out
As this is only a problem with the NS front, brake, I'd hazard a guess at the solenoid valve for that brake is sticking in the ABS hydraulic unit and not allowing the pressure to drop sufficiently, causing it to bind. You won't be able to replace the solenoid though, it'll be a non serviceable part, so would need an ABS hydraulic unit
How often do you change the brake fluid in your car, or put another way, when was it last changed? Brake fluid is hygroscopic, meaning it naturally pulls in moisture - moisture is water, water leads to corrosion, corrosion can cause hydraulic solenoid valves to stick.
Try this:
When the brake is sticking on, if you open the bleed nipple on the caliper, does the pressure drop, and the wheel become free?
It would also be worth cracking open the bleed nipple on the OS rear brake with the NS front sticking on - if that frees off the NS front brake, then it would make me think master cylinder
If it was one piston in the master cylinder sticking and holding pressure, I'd expect to see an issue with the opposite rear corner as well as the front as it will be supplying pressure to NSF and OSR so in this case, I'd rule it out
As this is only a problem with the NS front, brake, I'd hazard a guess at the solenoid valve for that brake is sticking in the ABS hydraulic unit and not allowing the pressure to drop sufficiently, causing it to bind. You won't be able to replace the solenoid though, it'll be a non serviceable part, so would need an ABS hydraulic unit
How often do you change the brake fluid in your car, or put another way, when was it last changed? Brake fluid is hygroscopic, meaning it naturally pulls in moisture - moisture is water, water leads to corrosion, corrosion can cause hydraulic solenoid valves to stick.
Try this:
When the brake is sticking on, if you open the bleed nipple on the caliper, does the pressure drop, and the wheel become free?
It would also be worth cracking open the bleed nipple on the OS rear brake with the NS front sticking on - if that frees off the NS front brake, then it would make me think master cylinder
Superhoop said:
davepoth said:
What are the brake flexis like? It's not entirely unknown for them to start breaking down inside and acting as a one way valve.
He says in his OP that they've been replacedbrake fluid not changed for quite a while but certainly not black. i've had the calipers changed less than a year ago so a partial bleed was done then.
can anyone recommend recon ABS pump companies?
Long shot, but how about damage to the brake line to that corner such that it will pass the high-pressure fluid from the master cylinder under braking but won't release the fluid back again very quickly, thus giving the impression that the caliper is jammed? A big dent in the brake line would do this I'd have thought, and would also fairly easily happen.
(I'm guessing here, and could be way out.)
Oli.
(I'm guessing here, and could be way out.)
Oli.
zcacogp said:
Long shot, but how about damage to the brake line to that corner such that it will pass the high-pressure fluid from the master cylinder under braking but won't release the fluid back again very quickly, thus giving the impression that the caliper is jammed? A big dent in the brake line would do this I'd have thought, and would also fairly easily happen.
(I'm guessing here, and could be way out.)
Oli.
yeah, that one occurred to me too. when it's in the air i'll do a visual check but from memory it was ok.(I'm guessing here, and could be way out.)
Oli.
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