Standard vs. performance brake pads

[mikeloadsasaabs]

I always like to have a go at doing the more straightforward jobs myself but I always worry when it comes to the brakes. If I loosen the bleed nipples to let out the excess fluid when I push the caliper back is there any danger I will get air into the system?

Not if you do the bleed nipples up before you release the pressure on the piston. TBH it's a bit awkward single handed, you can end up with brake fluid everywhere! And last time I did find that the brakes were a little spongy afterward, so perhaps I did get a bit of air in. But I did them only a couple of days before the MOT and I got the garage to do a complete fluid change anyway (as I do every other MOT), which firmed them up nicely.

[sgould]

Not if you do it properly and put a tube on the nipple and dip it into a pot of fresh fluid.  That way once the air has been forced out of the tube, you can't suck any back.

[mikeloadsasaabs]

Not if you do it properly and put a tube on the nipple and dip it into a pot of fresh fluid.  That way once the air has been forced out of the tube, you can't suck any back.

True, but I find that I could really do with an extra pair of hands when I do this job - the tube always seems to twist out of the fluid as the bleed nipple is turned......

[sgould]

Twist of copper wire was the old way....

[pompey]

Well if the right way is to let the excess fluid out through the bleed nipples then I'll get looking into some of those "one-man brake bleeding kits".

Thanks folks.

[sgould]

There are some kits that use a pressure system that tries to mimic the "professional way of filling the braking system by forcing fluid back from the callipers to the master cylinder and reservoir.  Not what you want.

If the kit is just the one that has the non-retun valve in the pipe.  You should be OK.

[pompey]

The ones I've been looking at are of the non-return valve type.

[sgould]

Should be fine.  I used them in the past, but owing to dermatitis, I can't do oily bits these days.

[pompey]

I can sympathise. My hands are very prone to eczema and it's not just irritating - it can be debilitating!

Another question... If I can't find out what brake fluid the garage used to refill the system 6 months ago am I safe to top up with any brake fluid as long as it's the correct DOT or is mixing brake fluids a strict no-no?

[sgould]

Saab specify DOT 4 brake fluid.  Any DOT 4 will do as a replacement.

There is a scope for confusion.
DOT 5 looks like a better type, BUT IT'S NOT, it's silicon based and doesn't mix with DOT 4.

ALTHOUGH DOT 5.1 will mix with DOT 4 because it's glycol based. 

The man who devised the coding ought to be taken out and given a good talking to....

[Mark B]

As a further clarification:-

The dry boiling point is the boiling point of the new (pure) brake fluid.

The wet boiling point is the boiling point of the fluid when it contains 3.7% (by weight) of water.  This is typical of a fluid when it has been in use for 2 or 3 years.

The viscosity limit is the MAXIMUM viscosity allowed at -40C.

DOT 5.1 has higher boiling points than DOT 3 or 4 fluid, but it has a lower low temp viscosity to ensure that systems like ABS, ESP etc work correctly when it's very cold.

Given that DOT 5.1 costs about £1 per litre more than DOT 4, I think it is worth using DOT 5.1 all the time.  It can be difficult to find but Halfords stock it.

Interestingly Castrol do not sell a DOT 5.1 fluid in the UK.  They sell a "Performance DOT 4" fluid, which has higher boiling points that the DOT 5.1 spec requires, but does not meet the low temp viscosity requirements.  Boiling points are 260C and 180C, low temp viscosity 1200.  They also sell a "Low Temp DOT 4", boiling points 265C, 175C and low temp viscosity 650, meeting the ISO 4925 Class 6 which prioritises low temp performance at the expense of a slight reduction in boiling point.

For further reading, in case of severe insomnia, see the following:-

https://law.resource.org/pub/us/cfr/ibr/005/sae.j1703.1995.html

http://www.shell.com/content/dam/shell/static/chemicals/downloads/products-services/shell-brake-clutchfluidsbrochure.pdf