I fired up the DLS (see: www.volvo300mania.com/forum-uk/viewtopic.php?f=2&t=9056) last weekend for the first time in 7 months and went for a quick spin around the car park to have a better "feel" for what the brake problem is (before that, I'd only ever driven it ten metres onto the back of the flatbed). The footbrake stops the car, but there is a lot of travel in the pedal before feeling the brakes bite.

Does that sound like the master cylinder's gone? If it was the brake servo I guess the pedal would be hard and difficult to push down, a bit like how the brakes go when the engine is turned off. Then there's also a kind of 4-way reduction valve, controlling how much pressure goes to the back (I think). Do those have a habit of going too?

The handbrake works perfectly, but I guess that means nothing as the cables work directly on the rear brake linings.

Cheers

Tom

Pump the pedal to firm it up and then hold it on. See if it sinks.

It may be a silly question but: with the engine running?

Cheers

Tom

Good question. Probably leave it off so there's no servo in the mix.

OK thanks mac. I'll give it a try and report back.

Cheers

Tom

OK, just did some testing. I pumped the pedal with the engine off. It firmed up a little, but not as much as I would have thought. Certainly wasn't rock solid. Then I turned the engine on, with my foot on the pedal, and it definitely gave a bit. Pumping it with the engine on didn't really make much difference.

Tom

trabitom99 wrote:OK, just did some testing. I pumped the pedal with the engine off. It firmed up a little, but not as much as I would have thought. Certainly wasn't rock solid.

Did you hold the pedal in the on position to see if it sank any further? (before starting the car)

Oops, I forgot I'll have another go later ...

Tom

Could this not also just be a perished pipe / needs an epic bleed? Though the dual circuited ness makes that seem unlikely I guess

Perished pipe's a possibility. But the car was main dealer serviced all its life, so it's not going to be something neglect-related like dodgy brake fluid or worn pads.

I had another go this morning. Engine off, pumped the pedal. This time I really can't say I felt anything firming up as I pumped it. Do that on a different car, and the pedal is rock solid after a few pumps, that's definitely not the case here. And since it wasn't rock solid it's pretty difficult to say, whether the pedal sank to the floor or not. I'd say it didn't - but I wasn't applying too much pressure to the pedal, as I would otherwise just have been compressing the pedal with the weight of my foot.

I've had a master cylinder fail spectacularly before on the old Merc I used to have, I wouldn't want to have that sinking feeling again It feels safe, still, the brake pedal is just very low before the brakes bite. It's not particularly hard to push though, which means I'm hoping it's not the servo. 360s (and Euro - "S" model 34x*s) have a different servo to 340s and you can't get them new anymore, unlike master cylinders. At least there's no LHD/RHD difference with the servos.

Cheers

Tom

OK, the car's at my local garage now. New brake master cylinder, the brake lines are all OK, the front discs and pads are good too. The pressure is a bit better, and the car will stop quickly (and in a straight line) if you push the brake pedal to the floor, but you can't make the wheels lock up no matter how hard you stamp on the pedal. Something's still not quite right ...

The garage reckons it could be the reduction valve / brake regulator thingy (Volvo 3298352 or 3294774). They're still available new and look like this:


http://www.teilesuche24.de/qh/bremskraf ... 9?c=100037

Does that sound likely?

I had a spare master cylinder, but I'd rather not splash out on something that's unlikely to have any effect

Cheers

Tom

trabitom99 wrote:but you can't make the wheels lock up no matter how hard you stamp on the pedal.

Are you really trying?

trabitom99 wrote:The garage reckons it could be the reduction valve / brake regulator thingy (Volvo 3298352 or 3294774)

I can't see that the reduction valve would make any difference. The PRV just prevents excessive pressure to the rear of the system which would otherwise make the back wheels lock up first under heavy braking. It's judged better that the front locks first and the car ploughs on in a straight line rather than the rear, which would make the back end a bit twitchy (in both senses)

What tyres is it wearing out of interest and how old?

Rather than this precise measurement,

trabitom99 wrote:The pressure is a bit better, and the car will stop quickly (and in a straight line) if you push the brake pedal to the floor

how about some numbers from either a decelerometer (Tapley) or a roller brake tester?

macplaxton wrote:

trabitom99 wrote:but you can't make the wheels lock up no matter how hard you stamp on the pedal.

Are you really trying?

Yes

macplaxton wrote:What tyres is it wearing out of interest and how old?

13" rubber which is still fairly fresh - a few years old at most ...

macplaxton wrote:Rather than this precise measurement,

trabitom99 wrote:The pressure is a bit better, and the car will stop quickly (and in a straight line) if you push the brake pedal to the floor

how about some numbers from either a decelerometer (Tapley) or a roller brake tester?

The garage had it is on a roller brake tester and the results were even too (apparently). I backed it up with a few emergency stops on a late night empty road

Cheers

Tom

The pedal normally firms up because you are using up the vacuum reservoir, the pedal should be hard otherwise you have got air in the system or air is being introduced. It could also be that you are not getting self-adjustment, but normally the handbrake would be an indicator of problems on the rears. Work the front pistons back in (as you would when changing the pads) and check the rear adjusters and general condition of the rears.

You still have a problem somewhere, you should absolutely be able to lock the wheels and the pedal should not travel to the floor. I also would doubt the PCRV would be at fault, they are just looking for easy things to change and guessing. It sounds to me like the problem was not the master cylinder and the bleeding the brakes to change it has improved it.

The brakes are best bled using a pressure based feeder (say from a spare tyre) pushing fluid into the system from the brake fluid reservoir, pumping the pedal can suck air in. You start at the rear left, then do the fronts. Put a full litre through it.

Thanks for all your help!

Ride_on wrote:I also would doubt the PCRV would be at fault, they are just looking for easy things to change and guessing. It sounds to me like the problem was not the master cylinder and the bleeding the brakes to change it has improved it.

Knowing the garage in question I can fully imagine they're just looking for quick and easy things to change. I may still order the part on commission though (getting the car to the garage is always a bit of a hassle and involves trailers and temporary plates), get the rear brakes checked and dismantled first (maybe a brake cylinder is leaking or something?) and send the PCRV back if the problem is sorted elsewhere.

It's worth bearing in mind that this was a well-serviced and regularly used car before the brake problem arose. I don't think the problem is due to neglect, or the car standing around for too long.

Cheers

Tom