V70: Wheels/Brakes Spongy brakes after Pad Change, only when engine running.

[Paulren]

Hi All,
Hope someone can help. Have just changed the front pads on my 2007 C70. Slowly compressed the pistons back with the reservoir cap off as I have always done. Everthing went fine and the brake pedal was solid initially but as soon as I started the engine, creating vaccum assist, the pedal went soft and I had to pump it. After doing this a number of times there was a small loss of brake fluid from the reservoir. When leaving the car for a while, allowing the vaccum to dissipate, the brake pedal returned to normal again, solid. Starting the engine made it spongy again. Tried the brakes on gravel and the ABS kicked in fine.
Have already done some research and aware that some people recommend opening the bleed valve when retracting the piston to avoid flipping the seal on the MS. Other people seem to regard this as an 'urban myth'.
I intend bleeding the brakes but not having opened the hydraulics at all, I am not optimistic of this solving the problem.

Any advice would be very welcome.
Paulren (new member)

[pierremcalpine]

I'm wondering if you have a leak in the brake line somewhere that was accelerated from pushing the piston back...maybe try getting someone to pump the brakes with the car on to see if you can spot some fluid dripping anywhere under the car. I too have heard that not opening bleeder can cause issues (potential for reversing master cylinder seals) but have seen lots of people with tons of experience chime in suggesting that they've never seen this happen. Good luck finding the root cause...please post when you've found it.

[tt82]

Just how spongy are the brakes? Modern braking systems rely on assistance of vacuum from the engine. If you press the brake pedal with the engine off and no vacuum assistance it will feel really hard and you won't be getting full braking power. (Try letting the car roll slowly and push the brakes with the engine off. The pedal will be hard but you will struggle to slow the car!) The fluid loss can easily be accounted for by the pistons moving back to their normal position sfter being fully compressed.

I would recommend a brake fluid flush, as they are suggested to be done every 2 years. That may help with spongy feeling. Unless you continue to lose fluid, I think the system is fine.

[BL08]

A simple test for a correctly operating vacuum assist is to press on the brake pedal and then start the car - it should go soft (or sink a little depending on the description) so that all seems ok.

As tt82 says, it is very likely that the pads will have settled back towards the discs causing a drop in fluid level in the reservoir.

Given that you've changed pads several times before without issue and you haven't opened the system to allow air in, I would have thought that things are ok from what you describe. I have not come across the seals reversing myself but I am aware it's a concern. A steady and careful road test will be in order bearing in mind that the new pads will need to bed in a little so your braking efficiency may not be quite what it was for a couple of hundred miles or so. It's also probably worth monitoring the fluid level to check that you're not losing any more fluid which assuming not, would lend additional weight to tt82's comments.

Good luck

[Paulren]

Thanks for all the comments. Have done a road test with a bit of left foot braking and things have improved considerably. Plan to do a system bleed which should get rid of any remaining sponginess. I suspect this was a bit of internet paranoia regarding the MS seal flip although this was apparently a real problem on Ford transits.
Thanks again for your comments.

[tt82]

Glad the system is returning to normal. A brake fluid flush should make a big difference.

[kenny]

This is not uncommon, i have come accross this several times on various cars old and new. I have asked various garages about it. No one can give a full explanation of what is causing it but all say the same thing, just drive it and the brakes will come back to normal in a few days and they always have.

[oragex]

My brakes got the same after replacing pads and rotors.

I did push the piston back without opening the bleeder.

I think it's a good idea to bleed all four corners when changing the pads, and to push back the piston with the bleeder opened. One reason is this will force out the old fluid that's inside the piston room. Then continue with gravity bleeding.

[Clan]

Originally Posted by Paulren

Hi All,
Hope someone can help. Have just changed the front pads on my 2007 C70. Slowly compressed the pistons back with the reservoir cap off as I have always done. Everthing went fine and the brake pedal was solid initially but as soon as I started the engine, creating vaccum assist, the pedal went soft and I had to pump it. After doing this a number of times there was a small loss of brake fluid from the reservoir. When leaving the car for a while, allowing the vaccum to dissipate, the brake pedal returned to normal again, solid. Starting the engine made it spongy again. Tried the brakes on gravel and the ABS kicked in fine.
Have already done some research and aware that some people recommend opening the bleed valve when retracting the piston to avoid flipping the seal on the MS. Other people seem to regard this as an 'urban myth'.
I intend bleeding the brakes but not having opened the hydraulics at all, I am not optimistic of this solving the problem.

Any advice would be very welcome.
Paulren (new member)

The problem is (assuming you have old discs ) new flat pads on old concave discs , the pad only touches the disc on inner and outer edge forming a bridge , when you press the brake the pad is forced into the depression ( It bends ) . Once the edges of the pad wear the whole pad surface will touch the disc and hence feel solid again .