Hi,
I've just replaced the brake pads on my Seat Alhambra and I now have a spongy pedal which sinks towards the floor when the engine is on but doesn't sink when the engine is off. Before anyone says it, I know all about servo assistance etc.

However, normally a sinking pedal indicates a master cylinder problem but in the case of a diesel engine which uses a vacuum pump instead of manifold vacuum to feed the brake servo, I was wondering if air in the hydraulic lines might cause a sinking pedal on a diesel due to the vacuum pump gradually providing more and more assistance as it compresses the air in the lines. If it was the m/c wouldn't it also sink under heavy (non servo assisted) foot pressure?

When pushing the pistons back I slackened the bleed nipples and bled off excess fluid rather than drive it back into the master cylinder so it is possible I let some air in. I can't shift it though using normal manual bleeding methods and I can't reach the m/c to attach a pressure bleeder without dismantling some of the engine bay. I did clamp one of the flexible hoses and this made the pedal quite a bit better so I am hopeful it is just trapped air, perhaps having moved up towards the top of the suspension turret.

Anyone had a similar experience?

Cheers,
Craig.

craig - this happened to me. think i reversed the seals in the master cylinder somehow.

Ned.

ps
http://www.locostbuilders.co.uk/viewthread.php?tid=38623

[Edited on 6/5/06 by ned]

Hi Ned,
I did consider that but if I clamp the flexible hose on one side it seems to make the pedal much better although not perfect. I've not tried clamping both front hoses yet but I suspect that may help further. This would indicate a problem at the caliper end.

As mentioned in your referenced thread, it could be the positioning of the springs on the ATE calipers although I have checked this several times.

I'll try a pressure bleed tomorrow and see what that does. Hopefully my Gunson's easybleed will have a suitable cap for the Alhambra...

Cheers,
Craig.

Sounds like you could have air in the calipers OR it could be that the pads need to be bedded in particularly if the brake discs are in poor condition/ scored etc. If bleeding with the eezibleed( which should fit ) doesn't restore pedal try (very carefully )driving the car and braking gently , pedal will sink but keep braking . pads should bed in . Do not allow car to stand with foot on trhe brake when discs are hot as brake discs could distort. HTH

its prob just the pads being new and the discs slightly worn,give it some miles and see what happens

Rusty,
I have been out for a drive around town and have done some light to moderate braking at junctions, roundabouts etc. I also went onto some gravel and braked hard enough to cycle the ABS system as I have read that this can sometimes help but no joy. The discs did have a lip on both sides at the outside of the disc which I rubbed down as best I could and then just bevelled the edges of the pads slightly to let them sit flat on the disc. The discs are still 24mm thick (min = 21mm, new =25mm) so they aren't bad and are not at all scored.

I'm pretty sure that there is air trapped somewhere but I think it is maybe not being expelled. Hopefully the pressure bleeder will fair better with the continuous flow.

By the way, do you know what sort of fluid I should be using in the car? It's a 2002 Seat Alhambra 1.9 TDI. The owner's manual says I should use "Genuine Seat Brake Fluid" but doesn't say what standard it is... I should be okay with something like Castrol DOT 4 shouldn't I? Seat don't use anything weird do they?

Cheers,
Craig.

My old A4 did it. if you (stationary!) stamped on th epedal VERY hard, it would sink to the floor after about 10 seconds.

It could still have stopped an express train though!

If youve done the pads and the brakes feel crap, give it some miles as conrod says.

Ed.
p.s. unless it was filled with silicone fluid (i will happily slam my willy in the drivers door if it was!) it will be fine with DOT4. The differnet DOT standards just boil at higher temperatures as the DOT number increases.

[Edited on 6/5/06 by ed_crouch]

Conrod,
I'm sure some miles will help but I am almost certain there is air in the system somewhere. The brake fluid that I have drained so far is pretty dirty and the car is overdue a fluid change so I might just get SEAT to do a complete flush for me. That way they can use their fancy pressure bleeder with ABS cycling and all that other good stuff to do a proper job. As long as it is safe enough to drive in the meantime...

Cheers,
Craig.

Bloody 'ell the replies are coming thick and fast now - I can't keep up!

Ed,
Yes the brakes are still very good although I'm not giving the full beans until they are bedded in a bit more. What worries me more than 40MPH to 0MPH stops is the higher speed stops where the pedal is under pressure for 5-10 seconds. I guess I'll just have to keep to lower speeds until I get it sorted...

Cheers,
Craig.

I'm still driving with mine as they were but they still work, when braking really hard i think only one abs circuit is coming on so still think there's a problem but the brakes are still as good as my other halves bmw but am gonna get it sorted before mot in a month or so for mot when i have the time (or hopefully have my locost on the road so i can use that for a feww days for work whilst the bora is in the garage )

Ned.

brake fluid is sposed to be changed every year or so,some ABS systems need a techneque ?,dunno about you seat tho

brake pedal creeping down is well known on diesel engined vehicles and has a specific section in the mot manual so long as it stops sinking without touching the floor it is acceptable. driveing, the brakes should feel normal. its to do with high boost servos used.

yeh TDI brakes are a bit funny cos theres not throttle butterfly and a turbo, hence no manifold vacuum to use, so they bolt a bloody great vac pump to the engine.

I did scare a guy once on the A3 heading into the 50 limit at tolworth, cos I saw a fridge lying in the outside lane ahead of me and had to throw the anchor out the window. He was too close and went through between me and the armco, sideways!!

It turned out to be a massive lump of polystyrene foam, but hey!

A4s/passats/octavias/etc do stop well!

Ed.

Thanks guys, I'll see how it goes. I suppose I was hoping that I would suddenly have a lovely firm pedal and plenty of bite after changing the pads (once they had bedded in of course). Maybe my expectation is part of the problem...

I'll try pressure bleeding tomorrow as planned since the fluid is (AFAIK) 4 years old and should be changed every two years. It may have been changed just before I got the car (2 years ago) but it doesn't say so on the service book so I doubt it has.

I'll let you know if this helps.
Cheers,
Craig.

http://www.brakesint.co.uk/technical.html#techNote202
knew i,d seen it somewhere

From the description that only leaks with the servo working it sounds like a fluid leak --- first check any banjo unions you may have on the caliper the frequently get disturbed when changing pads -- there is a known problem on VW group cars with problems with the alloy sealing washer they used on banjo fittings.

[Edited on 7/5/06 by britishtrident]

BT,
I had also considered this possibility but I didn't disturb any connections and there is no fluid leaking at the caliper and the master cylinder level is not dropping. The car is a 2002 model so the brake lines are still in good shape with no rust or perishing.

Off to Halfords shortly for some fluid to begin the pressure bleed.

Cheers,
Craig.

Hi,
Just a quick update:

I've pressure bled the system quite thoroughly and I think it is better than before but I still feel that the pedal is too long. The braking efficiency of the car seems fine and there is plenty of reserve pedal travel so I believe it is safe to drive. I think that part of the (perceived) problem is the way that the vacuum pump works on a diesel engine where under constant pedal pressure it generates more and more hydraulic pressure due to increased vacuum force and the pedal sinks. The fact that it sinks must indicate either a hydraulic or mechanical "softness" due either to air in the system or mechanical movement but when testing the brakes on the move you naturally ease off the pedal to maintain constant retardation as the vacuum pump increases its assistance.

So I'm going to see how it goes over the next week or so and if I'm still not happy I will take it to my SEAT dealer for a complete flush of the system.

Annoyingly, while I was bleeding the back brakes I noticed that my entire rear silencer has broken away from the centre section and will need replacement. The weird thing is that there is no increase in noise which makes me wonder what on earth the back box is supposed to be doing! The amount of soot under the heat shield suggests it has been broken for a while and I don't think I would have noticed if I hadn't bumped it while bleeding the rear brakes and realised that it was only attached to the vehicle by the rubber mounts!

[Note to self - sell future vehicles BEFORE the warranty runs out...]

Ho hum!

Cheers,
Craig.