Long brake pedal after brake refurb

[ric355]

I know the first thing most people are going to think is 'air in the master cylinder' but bear with me on this while I explain...

I have carried out a bunch of work on the brakes on my 450;
- Refurb both front calipers with new piston and seal kit (I had a seized one on the front left)
- Replace brake servo with one from TVR Parts
- Replace master cylinder with an XR2i one with 22.2mm bore 2xm12 2xm10 which I believe is the correct one. It matches the one I took off.
- both pressure and vacuum bleed of the brakes all round, multiple times, to dispel any air. Including bleeding the master cylinder via the one bleed nipple (only does one of the circuits of course).

I haven't done anything to the rear brakes or replaced any disks or pads.

I have the following situation;
- Engine off, brake pedal is hard as expected
- Start engine with foot on the brake, pedal goes down so servo is working as expected
- Pedal travel is reasonable and pedal feels hard at the right point i.e. not spongey. Brakes aren't 'pumped up' by repeatedly pressing the pedal
- When I drive the car, the pedal travel gets longer. Brakes still work, and they *can't* be pumped up. It's just that the point it starts to bite is a lot further down once you move it. And you don't have to move it very far for this to be evident. Reversing out of the parking place is enough for it to be noticeable and then as you drive it another 20 metres or so forward it goes long.

I spoke to TVR parts about the brake servo, to check that behaviour should be like for like with the original, and they confirmed that this is the case. I am pretty convinced I have got all of the air out of the system, because the brake pedal is not spongey. I have also confirmed that the brakes start to be applied at a point near the top of the pedal travel, both on the front and rear, when the car is on the lift. It is hard to check the same is true when driving it, but coasting with the clutch disengaged and lightly touching the brakes does seem to have a slight slowing effect. But proper braking still does not happen until a lot more pedal travel has been applied (at which point at the end of that travel I have a supportive pedal i.e. it doesn't feel like the brakes aren't working properly).

One of the things TVR Parts suggested was adjusting the threaded part of the clevis on the pedal. I put it back basically where the original was and although I'm not averse to trying this it just doesn't seem like it would have the right sort of impact. It would only change the static height of the pedal, not the travel of the servo rod in relation to the pedal movement. But please correct me if I'm wrong on this. It would need to make several inches travel difference to make up for the long pedal and anyway changing this does not match the symptom of "OK until you drive it".

The brake servo also has an adjustable nose. I set this so that there is a small amount of clearance between it and the master cylinder piston. I can feel this takeup on the pedal at the top and it's just what you would expect. Easiest to feel with engine off. So I think it is adjusted properly and again this adjustment does not match the symptom.

So the question is what would cause the pedal travel to suddenly get longer after moving the car? After much thought the only thing I can think of is pad knock back. Front discs have been off the car and I took the time to remove the rust on the insides and the hub faces so that they would sit flat. I don't think I have a bad wheel bearing or anything. I can't feel anything and car passed its MOT a couple of months ago. If the master cylinder were the wrong one, then I would always have too long pedal travel, not only sometimes. I have not measured thickness of the discs as I forgot last time I had the wheels off, but pads are 17mm thick (new is nominally 18mm according to data I found). Again, if it were purely down to pad or disc thickness pedal travel would be long all of the time.

Does anyone have an alternative hypothesis to pad knock back? Or has anyone had a similar situation? It could still be air but it just doesn't feel like it. If it is pad knock back then I'm not entirely sure how to fix it unless something is misaligned. I can put an indicator on the discs to check the runout I guess but I can't do it with the car on its wheels which is when the problem manifests.

Edited by ric355 on Tuesday 8th December 14:16

[Oldred_V8S]

Bear with me.

Have you fitted the brake calipers on the correct side so the bleed nipples are pointing upwards.
Sorry if this seems a bit of a dumb question but I gave the advice to someone many years ago who replied that he wasn't stupid and tore me a new one but after checking later came back and apologised.

With the calipers on the wrong side you will get fluid out of the nipples, but leave a large bubble of air in the top where the bleed nipple should be.

[ric355]

Yes, nipples are pointing upwards and in fact I only did one side at a time so couldn't mix them up as while doing one the other was still on the car.

[roseytvr]

From memory I had a similar problem that was solved by jacking up the rear of the car when bleeding the master. If I remember correctly the cylinder points up and can trap air past the bleed nipple.
Also check there is no air leak in the servo vacuum. Good luck

[Steve_D]

Does sound like you still have air in one of the systems most likely the rear. Your long pedal is the master pushing past the air in one system and going hard against the good system.
The rear can be a right pain to bleed due to the compensating valve being very restrictive.

Steve

[ric355]

Steve_D said:

Does sound like you still have air in one of the systems most likely the rear. Your long pedal is the master pushing past the air in one system and going hard against the good system.
The rear can be a right pain to bleed due to the compensating valve being very restrictive.

Steve

Interesting. I did notice that there wasn't much in the way of fluid coming out of the rear, although none of it had bubbles in it. And I think the rear circuit is the one with the spare port that has a bleed nipple in it on the master cylinder?

Is it better to bleed it with vacuum to the servo or without (I did it without, engine off). I used pressure mostly but also tried a non-return valve on the rear which seemed to be working ok in the sense that it maintained fluid in the bleed tube when I lifted the pedal.

What I don't understand though is the fact that I can get a good, solid pedal with the engine on (goes down to around the throttle pedal position then goes hard), and it stays there no matter how many times I press it, but as soon as I move the car even only a few feet I lose it and the pedal goes long.

[LLantrisant]

i would say there is something not ok with the servo or the pin (adjustable?) of the pedal which acts on the servo.

during idle you have a certain vacuum...when the car is moving the vacuum changes slightly and maybe thats the differnce.
just an idea.

but before check again bleeding of rear...try by vacuum or by pressure on the reservoir

[Matt_E_Mulsion]

Has everything bedded in yet? I replaced the discs and pads on my Vito van and the pedal travel felt terrible for a week or two afterwards, even with daily use. I thought I'd messed something up in the process even though I've done brakes a hundred times. But once it had done some miles the pedal feel and travel was back to what it was before and is now spot on.

[ric355]

Matt_E_Mulsion said:

Has everything bedded in yet? I replaced the discs and pads on my Vito van and the pedal travel felt terrible for a week or two afterwards, even with daily use. I thought I'd messed something up in the process even though I've done brakes a hundred times. But once it had done some miles the pedal feel and travel was back to what it was before and is now spot on.

Discs and pads not changed.

[x 7usc]

Ive had this on my old Tuscan, caused by a wheel bearing/hub failure producing pad knock back like you're suspecting. Sat on its wheels no play could be found on the wheel at all and driving felt normal (other than the same pedal travel issue) it was only when the car was elevated with the wheel on it showed up as a movement on the wheel as it left the ground, from memory even pulling on the problem wheel you couldn't actually feel a lot of play. Are you sure the disks are sat flat against the hub on a smear of copper grease, highly unlikely they aren't but perhaps worth checking all the same

[ric355]

x 7usc said:

Ive had this on my old Tuscan, caused by a wheel bearing/hub failure producing pad knock back like you're suspecting. Sat on its wheels no play could be found on the wheel at all and driving felt normal (other than the same pedal travel issue) it was only when the car was elevated with the wheel on it showed up as a movement on the wheel as it left the ground, from memory even pulling on the problem wheel you couldn't actually feel a lot of play. Are you sure the disks are sat flat against the hub on a smear of copper grease, highly unlikely they aren't but perhaps worth checking all the same

I cleaned the back of the discs up and the face of the hub before reinstalling, so it should be flat.

I will measure the runout tomorrow on all 4 corners if the weather allows.

[ric355]

Update;

I checked the runout on all four disks this morning and they seemed reasonable. The worst was about 0.3mm; I've never been able to feel that under braking. All the rest were in the 0.05 to 0.1mm range.

I then bled the master cylinder again (big syringe). And then bled all four corners using an ez-bleed at about 15psi and with an empty bottle (manual top-up) as I find it is a lot less messy that way. I bled the rears for a lot longer this time. I didn't see any real evidence of air in the lines.

On starting the engine it felt exactly the same as before but this time on the test drive the pedal stayed high.

So one of the things I did fixed it but I don't know which. If I had to guess I'd say it was either the syringe on the master cylinder (which I had done before), or it had self bled back to the reservoir after sitting for a couple of days without me messing with it.

I've still been unable to explain why the pedal was high until I drove it, but it may just have been that the engine vacuum at idle doesn't give full assistance i.e. a slight difference in the brake servo operation compared to the old one. It is a different design to the original after all.

[tofts]

Your problem is air in the master cylinder and it is literally impossible to solve completely when its on the car. You end up with a TINY amout of air in between the 2 pistons inside which can not escape because of the presence of even pressures boths sides. the only way to solve is to bleed it on the bench before installation.

You can chase this issue forever and unless you take it back out to bench bleed it, you will likely never be able to solve it completely.

Been there seen that got the T-shirt, never again will that mistake be made.

Edited by tofts on Thursday 10th December 00:10

[hoofa]

I have the same issue after fitting new servo and master cyclinder, so,watching this with interest. I should have fitted a remote bleed on the master cyclinder when it was installed

[ric355]

hoofa said:

I have the same issue after fitting new servo and master cyclinder, so,watching this with interest. I should have fitted a remote bleed on the master cyclinder when it was installed

You may have missed the post up there where I said I fixed it ^^^