Coming home from work this morning I noticed that after braking into a roundabout my brakes were binding as I pulled away. The pedal went rock solid and the brakes appeared to still be on. I tapped the pedal and they freed off.

Calipers are brembos that have recently been refurbed and the one way valve appears to be working okay and I am assuming it is the right way around in my aftermarket hose; fitted so you can't blow down it into the servo. The pads are used but don't seem excessively low although I am about to change them.

Looking for any ideas as to what it could be. The pedal does go hard occasionally but I have never had the brakes lock on before.

 

pad or caliper binding/sticking??

if not then i presume you are talking about the one-way valve in the vaccum pipe from the inlet, this should allow flow on idel to the servo (to assist your braking) but close as you open the throttle to "store" air in the servo, sounds like you have it back to front

alyn

 

pad or caliper binding/sticking??

if not then i presume you are talking about the one-way valve in the vaccum pipe from the inlet, this should allow flow on idel to the servo (to assist your braking) but close as you open the throttle to "store" air in the servo, sounds like you have it back to front

alyn

 

Alyn, wrong way around.

The servo is vacuum assisted i.e. air is drawn out of the servo by the depression in the inlet manifold and a vacuum is held in the servo by the non return valve whilst on boost.

That's why its always difficult to remove the servo pipes as most of the time they are under -ve pressure.

If you hit the brake pedal a couple of times with the engine off, the vacuum is lost and the pedal goes hard.

http://en.wikipedia.org/wiki/Vacuum_servo

 

Sounds like a servo issue Jon if the pedal is going rock hard?

 

yep sounds like brake servo,had same on a bmw i had. also had brake pads sticking on hard, turned out pads were very worn allowing the calliper pistons to travel out to far causing them to stick in forward position against disc. rock solid pedal - vacume servo. u can test the servo by pumping pedal with ignition on then hold pedal down turn ignition off, i think something like that.

 

Confused now, I read Alyn's post and thought yeah - simple fix, but it appears I may have it the right way around

I've done the static ignition on/ignition off pumping the pedals test and it seems okay. The pads are used but still have 4-5mm of material on each pad. I've got new pads to fit so I'll get that done and see what difference it makes. Could be sticking pads but not calipers - they were rebuilt 6-months ago with all new genuine seals!

 

That's what I was thinking - picked up a second-hand one for less than £20 so if changing the pads doesn't make a difference I'll fit the new servo

 

Jon,

Easy test..........

Put foot on brake pedal and push gently -should be fairly firm.

Start engine and pedal should immediately) move easily under created vacuum assist.

If it stays firm then servo not working for some reason.

 

That works fine, also if I pump the pedal with the engine on the pressure remains constant - this issue is an intermittent problem whilst driving. Usually the brakes are fine, but every now and again the pedal goes hard. Last time I drove it, after one of the these incidents when the pedal went hard the brakes were binding but as soon as I pumped the pedal they released - new pads and a good clean is the first step I think

 

Happened again this morning - once after about 10-miles and whilst I was looking for somewhere safe to pull over it released - no issues again all the way into work. If it was the servo surely this would not be intermittent. I even tried switching the engine off and restarting and it made no difference.

The valve is set so that if I blow it does not fill the servo, but if I suck through it I can draw air out and maintain that vacuum.

Pads not changed yet, fronts will get done Friday and rear with new discs if I get time as I'm working nights. I will start to take a screwdriver to remove the hose to see if releasing the vacuum makes a difference too

 

Question: If the servo was knackered I understand the brakes going hard as they would loose the assistance - but would they lock on?

Apart from the valve between the servo and the manifold are there any other valves in the system that could cause the system to lock and pads not to release?

 

Do you have a little free play at the top of the brake pedal?

 

Not that I've noticed - does that lead to something specific

 

I defo think it's servo related Jon,either you have a problem with your one way valve or the servo itself is playing up.

 

ABS??

Proportioning valve.

Brakes
http://ken-gilbert.com/wrx/01impreza/imp01_chas_9.pdf

ABS
http://ken-gilbert.com/wrx/01impreza/imp01_chas_7.pdf

ABS - Diagnostics
http://ken-gilbert.com/wrx/01impreza/imp01_chas_8.pdf

 

One way valve is relatively new and seems okay - can use the mrs car over the weekend for my nights so I can start to rip it apart and buy some brake fluid ready to top-up after the servo change and see how it goes from there.

 

thats what holly mode does...............teach me to try and have a few days to myself

 

If it is it'll be time to remove it

 

Noticed when I got home tonight that the drivers side front caliper was warmer than the rest. It got home without seizing tonight but the pedal still went hard. I'll do the pads and discs and then bleed the calipers to see if that changes anything. Then I'll do the servo, failing that I'll get the ABS ripped out and an adjustable proportioning valve fitted

 

You actually need a little free play. If not its similar to driving with your foot lightly applying the brake, therefore building up both heat and pressure and creating problems just like you describe.

 

What you're describing is there until the brakes go solid - I'm hoping that what I've discovered is that the drivers side pads are bind a touch, maybe due to wear, and changing the pads will fix it

 

Best thing to do Jon is change the pads and give it all a good clean and try it then,could be due to the mile munching it has caused an excess of **** build up around where the pad slides on in the calliper.

 

That's the plan mate, I don't expect the calipers to be seized given the Brembos have alloy pistons and the fact that you fitted the new seals when you refurbed them. A light bleed won't hurt. I'm after removing the ABS anyway, just need to get a new proportioning valve and find someone who can re-route the brake lines. I need to find the 'how to' on NASIOC too

 

It won't be the pistons seized for sure,it could well be a pad sticking though with how many miles you have been doing lol!
I am considering removing the ABS but with my potential power output and being a classic with big brakes it may well prove a disaster in the making to not have it

 

Just need to learn to drive without using the brakes

 

Yeah use the throttle to control the car

 

the fact one caliper is hotter than the rest would sugggest its "seized" in some way be it pads or pistons.............

this would also expalin the hard pedal as when the pad massively overheats it will "fade" and potentially cause the symptoms you describe

 

Still waiting for the rear discs to arrive - but swapped out the front pads today and the drivers side front was definately binding. When I tried to remove the pads the inner pad was seized solid whilst the outer one slipped straight out. Once it was out all of the pistons checked and all free. It looked like the stainless 'spacer' was a little bent but the star drive screw was stripped of its star so a hobby file was used to clean it up. New pads in and all seems better straight away - no more binding and the temps across the calipers are now all equal

 

Good news Jon,I did try and get them out when I refurbed them mate but they wouldn't budge,most brembo's/k-sports and AP's are all the same with that shim holding screw