Volvo Owners Club Forum - View Single Post - V70: Wheels/Brakes - Spongy brake pedal after changing pads and disks

green van man · Dec 13th, 2017, 17:10

I have heard about this rubbers flipping when you push the piston back thing, not convinced myself unless the master cylinder has a tapered bore.
The fluid pushing back into the master cylinder is the same as the master cylinder pushing the fluid out.

I confess I have never had cause to strip a Volvo master cylinder, all the ones I have, come with one or two seals on the piston dependant upon single or twin circuit, neither of which have room to flip when fitted to the cylinder, if they did have they would certainly flip when brakeing.

A spongy pedal can be caused by the new pads not being bedded in, long travel can be caused by the disk not being fitted square. Did you grease/ copper slip the mateing surfaces of the disk and hub? If so this may be the cause of your problems as it can cause the disk to be mounted out of square and give pad knock back, you then have to make up this extra clearance every time you brake giving you long peddle travel. Disks should be fitted to the hub dry, it is OK to grease the mounting spigot on the hub lightly to prevent the wheel sticking.

Quick test of a servo is to pump the peddle with engine off until it goes hard, start engine and peddle should move towards the floor as vaccuum biulds. It's a test I perform regularly on my vehicles, a longer time to build vaccuum than usual indicates a failing vaccuum pump or some other issue that needs investigation.
I change my brake fluid every 2 years so should have no moisture ingress problems but old fluid that has absorbed moisture can also give a spongy peddle.

It looks as if it's something you've done or the parts you've fitted that has caused the problem as it was fine previously. If you greased the hub/disk mating surfaces I would be stripping them and washing the grease off with brake / clutch cleaner or petrol, if not I would be stripping them and checking carefully every component as I rebuilt it.

If it comes to stripping the master cylinder then I would be looking for a service pack of new seals rather than a new cylinder if possible, it's a Volvo, they want £85 for a disc, eyes will water at the price of a master cylinder.

I most certainly would not be handing it over to a garage at this point, if you can find where you went wrong you will never make the mistake again, have someone else fix it and you will never learn.

Paul.

Dec 13th, 2017, 17:10	#8
green van man Premier Member Last Online: Apr 11th, 2024 09:21 Join Date: Nov 2012 Location: Ffos y Ffin	I have heard about this rubbers flipping when you push the piston back thing, not convinced myself unless the master cylinder has a tapered bore. The fluid pushing back into the master cylinder is the same as the master cylinder pushing the fluid out. I confess I have never had cause to strip a Volvo master cylinder, all the ones I have, come with one or two seals on the piston dependant upon single or twin circuit, neither of which have room to flip when fitted to the cylinder, if they did have they would certainly flip when brakeing. A spongy pedal can be caused by the new pads not being bedded in, long travel can be caused by the disk not being fitted square. Did you grease/ copper slip the mateing surfaces of the disk and hub? If so this may be the cause of your problems as it can cause the disk to be mounted out of square and give pad knock back, you then have to make up this extra clearance every time you brake giving you long peddle travel. Disks should be fitted to the hub dry, it is OK to grease the mounting spigot on the hub lightly to prevent the wheel sticking. Quick test of a servo is to pump the peddle with engine off until it goes hard, start engine and peddle should move towards the floor as vaccuum biulds. It's a test I perform regularly on my vehicles, a longer time to build vaccuum than usual indicates a failing vaccuum pump or some other issue that needs investigation. I change my brake fluid every 2 years so should have no moisture ingress problems but old fluid that has absorbed moisture can also give a spongy peddle. It looks as if it's something you've done or the parts you've fitted that has caused the problem as it was fine previously. If you greased the hub/disk mating surfaces I would be stripping them and washing the grease off with brake / clutch cleaner or petrol, if not I would be stripping them and checking carefully every component as I rebuilt it. If it comes to stripping the master cylinder then I would be looking for a service pack of new seals rather than a new cylinder if possible, it's a Volvo, they want £85 for a disc, eyes will water at the price of a master cylinder. I most certainly would not be handing it over to a garage at this point, if you can find where you went wrong you will never make the mistake again, have someone else fix it and you will never learn. Paul.