Changing brake fluid

[Mr Spigot]

Having read with interest the numerous posts on DOT4 versus DOT5, as the brake fluid in my M1000 is due for a change, I had decided to invest in some DOT5 silicone fluid and flush out the old DOT4 stuff when the car gets its Spring service shortly. However, having started the older MM today to bring it home to the garage, I discovered that the brakes were stuck on. Mental note to self - remember not to leave the handbrake on if the car is not being driven for a while. After 45 years playing with these cars, I really should know better. So once I have checked and freed the brakes, it makes sense to replace the brake fluid in both cars with silicone, which leaves me with 2 questions:

1) How much fluid should I get to do both cars? The MM system is standard but the M1000 has disc brakes and a remote reservoir under the bonnet. As far as I can see, the systems require about 0.5 litres each, but I wondered if I might need a little more to ensure all the old fluid is flushed out; which leads me to…

2) I have read somewhere that you can flush the system with methylated spirits to clean the old mineral fluid out. Does this work? Is it something “my mate down the pub…” said? Will the new fluid suffice or is there something else that would work better.

Any other advice? I await the voices of the wise….

[svenedin]

I changed to Dot 5 a few years ago (no problems with seized brakes since) but at that time I had replaced the entire brake system.

According to the info on this page (Penrite Dot 5) it is sufficient to flush through with the new Dot 5 fluid:https://penriteclassicoils.co.uk/produc ... ake-fluid/

Dot 5 is really expensive but it does not attract water so it has an almost indefinite shelf life. There is no problem if you order a bit too much because you can always use it for top ups if you need to bleed the system in future.

Remember to label the master cylinder and any remote reservoir "Dot 5 only!" so that nobody is tempted to top up with the wrong fluid.

Stephen

[King Kenny]

I second all that Stephen says. I replaced the complete breaking system in the 1990's and have never had a seized cylinder since. I stress that all brake lines, flexables, wheel cylinders and master cylinder were changed or stripped and rebuilt with new seals. I fitted disc brakes at the same time. I think any the advantage would be lost if you try to flush the system and not replace it. If you do this then a one litre bottle should be plenty. I have filtered used fluid through a coffee filter and use it again.

[philipkearney]

There should be no problem with re-using the fluid if it is well filtered, but certainly with Dot 4 fluid, you need to leave it to sit for about 24 hours to allow any air to escape before reuse. I'd imagine DOT 5 would be the same. Given that all pipes and cylinders will need bled, you'll need sufficient fluid to the fill the system on completion and potentially enough to allow bleed of the system, possibly 30-50% extra, or bleed the fluid through, allow to settle and then cut ntinur with bleeding. As others have said, any extra can be stored indefinitely so is not wasted.

[Mr Spigot]

I have no intention of putting back the old fluid but want to replace it with silicone fluid.

[svenedin]

I have no intention of putting back the old fluid but want to replace it with silicone fluid.

Yes so just bleed the system dry of the old glycol fluid and replace with the Dot 5. The Dot 5 is an interesting violet-purple colour and very obvious as it runs through. You’ll get air in the system and this is more difficult to bleed than with glycol apparently. You may need to use some tricks to bleed successfully like jacking up the back of the car and leaving overnight etc. There is no fear of Dot 5 hurting paintwork so you could even open unions to help with the purge. It is said that Dot 5 can give a slightly spongy pedal feel as it is more compressible than glycol based brake fluid but I can’t say I have noticed a difference personally.

Stephen

[Mr Spigot]

Thanks Stephen. I will drain it dry and refill as suggested.

[philipkearney]

I have no intention of putting back the old fluid but want to replace it with silicone fluid.

Sorry, I have may have confused you. I didn't mean re-use the old fluid. When you bleed through the new silicon fluid, the fluid you capture from the bleeders in the bleeding bottle can be reused after being left to settle and the air escape. You don't need to dispose of it.

[svenedin]

I have no intention of putting back the old fluid but want to replace it with silicone fluid.

Sorry, I have may have confused you. I didn't mean re-use the old fluid. When you bleed through the new silicon fluid, the fluid you capture from the bleeders in the bleeding bottle can be reused after being left to settle and the air escape. You don't need to dispose of it.

Yes but I would throw away the first part of the new fluid at every bleed nipple as it could be mixed with the old fluid.

Stephen

[philipkearney]

Agreed, but obviously not necessary if the system is new.