Long Brake Pedal Travel After Reassembling Braking System from Dry, Pumping Doesn't Help

Technical Area Wheels, Tyres & Brakes
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  1. My model of MX-5 is: 2004 Mk2.5 with ABS
  2. I’m based near: Ashford
  3. I’m looking for technical help or recommendations on: Long brake pedal travel.

I’ve read 10’s of other forum posts about this sort of thing, but couldn’t find anything matching the symptoms I’m seeing.

Immediately after bleeding the front brakes, the pedal will be hard on the first pump. All subsequent pumps quickly reduce travel until I’m stuck with heaps of travel, and then a hard pedal just before it reaches the floor. Pumping the pedal doesn’t regain any travel, nor does leaving it for half and hour.

All corners bleed ok. The rears retain the reduced travel, so take forever to pump fluid through. The fronts appear to regain pedal travel as they’re bled, so after a couple of pumps can start to really shift fluid through them like I’d expect them to.

When bleeding I’ve had the hand brake on and off (doesn’t seem to make a difference), and the discs are held on with 2 nuts, so can’t move.

For some context, the car’s been up on axel stands since around April last year, and for maybe 3-4 of those months, the braking system’s been completely to bits off the car, and empty of fluid. When reassembled, the rear calipers were new, the front calipers have been cleaned off, sliders greased, and had seals replaced. Rear lines have been replaced, front lines are still the old ones.

The whole thing was bled from empty. I started by pulling fluid through from the reserviour with a syringe on the caliper (pulling the syringe to suck fluid into the syringe from the reserviour). Once there was something in each caliper, I switched to pumping fluid through with the brake pedal. There are no bubbles, and there must’ve been about 500ml of fluid through each caliper at this point, so I’m hoping that there isn’t some air that’s still hiding in the lines.

When that failed, I did some reading and realised I hadn’t re-adjusted the rear caliper screw. Did this, then re-bled. No improvement seen.

After more reading, it turned out I should have bled the master cylinder outside of the car before refitting it. So I took it off and bled it in a vice. A little air came out, but nothing much at all. Blocking up the two pipe holes with fingers (with rubber gloves forming a seal) made it impossible to push the piston in by hand.

Refitted the MC, then re-bled each corner, going Rear Left, Rear Right, Front Right, Front Left, as previously done. About 500ml went through the system, so say at least 100ml per corner.

This gave no improvement either.

All I can think of now is that either I need to do even more bleeding, the master cylinder (what with it being dry for so long) has gone past the failing stage and has jumped straight to completely failed (although if that was the case, wouldn’t the bench bleed have shown something up?), or maybe some junk’s got into one of the brake pipes when everything was apart and is causing issues somehow.

2

It might be worth clamping each flexi hose one at a time and trying it to see if you can narrow down which (if any) corner is causing the problem.

3

How long was she sat dry for?
I am going for the master or air in the ABS unit, but saying that I have just sorted a MK 2.5 svt with the so called large sports brakes that had braking issues for a local owner, which turned out it had a slight weep on a line under the body of the Mx, once that was cut out and a new piece fitted , we always have the ABS mx’s running when we bleed off or bridge the abs pump to work to vacuum draw through the system.

M-m

4

The rear brakes have been off for about 12 months, but not quite all the fluid had left the master cylinder when I came to take that off about 4 months ago. So anywhere between 4 amd 12 months it’s been dry.

I’ll give the abs a go tonight and see how that does.

5

Still no luck.
Bled the brakes with the engine running, and every 4-5 pumps would put it in gear, then apply the brake lightly. The first time I did this, the ABS light came on, along with the brake warning light. I then completed the other 4 corners (2 sets of 4-5 pumps each), the brake warning light and ABS light stayed on for the duration (if I put the hand brake on, the brake warning light came on ‘more’ (a little bit brighter)).

Upon restarting the car, the ABS light stayed on, but the brake warning light went out, and hasn’t come back on since.

As far as the brakes go, there’s no improvement. I tried pushing fluid back up from the calipers to the cylinder with a large syringe, but couldn’t get anything in with moderate pressure on the syringe.

All ABS sensors measure about 1.6kOhms stationary, and if I spin the wheel with my foot, the resistance increases, so I’m assuming they’re all good. The ABS fuse is fine too.

I thought maybe I’d adjusted the right rear caliper too tight, and this had caused a wheel to lock, which may have triggered the ABS light, because the right rear had siezed (couldn’t turn it with a breaker bar as a lever between wheel studs). Did it up and then backed off 1/3 turn and it spun ok again. Let the car run for a bit in gear with the wheel spinning in the air, but ABS light didn’t go out.

Tried to look for codes with a code scanner, but nothing came up.

So now I either have 2 problems, or 1 problem that’s causing 2 symptoms. Long pedal travel, and an ABS light that won’t go out.

I’ll get a used master cylinder on order this evening, and see if that solves the brake travel problem. One thing at a time…

6

I thought you had to put the ABS modulator unit into a special cycle mode to bleed air out of them.

7

The ABS light maybe illuminated because the battery has been disconected and the steering wheel position sensor has not been re-set is it? Presumably the car has not been driven in the last 12 months?

8

Looked at the codes at lunch today, got 41, 42, and 44 (both front ABS sensors and the rear left). Guessing this is the system getting in a muddle because it was up on jack stands.
I read about the diagnostic mode for the abs unit, and will try it out today.

9

So it turns out that clumsiness and idiocy was the issue…

Cleared the ABS light by shorting TBS+GND, letting all the codes read out, then pressing the brake pedal over and over again until the brake warning light came on (it was 11 or 12 presses that seemed to do it), then turned ignition off, and when it was turned back on, the ABS light was off.

Then did the same bleed process as before (twice around the car), but running the ABS pump with the caliper bleed nipples closed after every 5-ish pumps of the pedal (short TBS+GND, then turn the key with foot on the brake pedal).

None of this seemed to make any difference.

I thought that (for lack of anything else to try) running the pump with foot pressed hard down on the pedal might help somehow (previously I’d been taking my foot off the brake before the pump started).

There was a rattle from around the pedal…

Perhaps I’d’ve noticed without the rattle if I’d been driving the car recently, but either way, it turns out that when I put the pin linking the servo to the brake pedal, I’d managed to put the pin through the servo, but not through the pedal. The two weren’t actually linked, but rubbing together. So the tons of travel I was feeling was the pin sliding up and down on the pedal leg, and the small bit of bite at the end of travel was when everything had run out of play, and the pedal finally started pushing the servo in.

Brake Fault
Brake Fault1042×590 6.91 KB

Put the pin back in correctly, and brakes were much firmer!

It now looks like there’s some weeping from around the front seal on the brake reservoir, and the pedal still goes to the floor quite easily when the engine’s running, but at least I can feel it pushing something about now when the pedal’s pressed.

Gave it another bleed and that didn’t seem to improve things, so the master cylinder still might be gone. Maybe all the spare bits I ordered yesterday won’t go to waste after all.

10

So have the codes gone and warning light off now ?
I still think it is air in the ABS pump.
M-m

11

Codes and warning light are both gone.
Any tips on how to get the air out, or should I keep running the pump and bleeding until something happens?

12

Changed front brake lines and the reservoir seals at lunch.
Jacked car up more at the back then bled each corner, running the ABS pump 3 times between corners.
Now, when the engine’s running the pedal goes to the floor, but when the engine’s off, the pedal feels pretty firm.
So it feels like some sort of progress at least.

13

Looks like (as multiple people and posts have said) bleeding the ABS is just a massive pain.
The brakes work just fine now and handled the drive to the MOT centre.

Some combination of the following must’ve shifted some air out:

  • 10ft back and forth on the drive, braking harshly now and then,
  • Cracking fittings on the 3 exit pipes on the ABS unig and bleeding from there,
  • Another couple of litres worth of bleeding and cycling the ABS pump.

I’m sure the feel could be better, but who cares! The MoT’s passed and once the DVLA send the logbook back, it’ll be taxed and back on the road.