Switching between ordinary brake fluid and silicon

[ricky8664]

Should be doing the final assembly of the braking system this week. However, as I've been unable to find my copper brake pipe set, I'm going to keep the old pipes for the time being.

This creates the conundrum. I do not want to fill with silicon fluid now as I know that once I find the copper brake pipes, I will put them on and have to drain out the silicon and the fill it up again.

So, can I fill up with cheaper standard brake fluid now and then refill with silicon once I have fitted the copper pipes? I seem to recall reading years ago that once the rubber parts had come into contact with ordinary fluid, they would absorb some of it and be contaminated, thus negating the subsequent use of silicon fluid.

If that's the case then, as all my brake rubber parts have been replaced, I should fill with silicon fluid, even though I know I'll be draining it out when I fit the copper pipes in the future.

So is contamination of rubber brake parts still an issue with standard fluid use?  

[Mike’S-a-loon]

Did you smear brake fluid on the cylinders when you reassembled them? If so, what kind?

[Phil Nottingham]

If you want Silicon fluid in then new rubbers are required from the outset as well as clean new brake pipes including flexis.  If you have lubed the new seals with ordinary fluid and used the old pipes then you must use mineral fluid to DoT4 spec never silicon.  

I really cannot see the point of using silicon despite the lack of corrosion that mineral fluid causes if not replaced regularly and it is said that the pedal is less firm.

So replace the steel and flexi pipes now if you must use silicon

[lagain]

I have been using silicone brake fluid for 30 plus years and it is far better than the corrosive stuff. It does not damage paintwork and it does not need changing every 2 years as it does not adsorb moisture. The brakes on my car are very sharp, but changing the pads made quite a difference and it can be difficult to bleed all the air out of the system.
It used to be said that the 2 fluids were compatible, but when I made the change, after replacing the brake pipes with copper and all the rubbers and the rear cylinders, I had problems with the servo and had to replace the complete unit.
The last time I replaced all the brake rubbers I used specific brake red rubber grease when reassembling

[enigmas]

You might find this Moss Motors tech article relevant in arriving at a decision as to which brake fluid to use in your classic car.

Conventional versus Silicone Brake Fluid

[ricky8664]

THe article was interesting but not conclusive either way.

As I have a set of copper brake pipes "somewhere" it's not practical to replace the steel pipes now as I'd have to go out a buy a new set of pipes. THat being the case I think I'll stick with standard Dot 4 fluid, even though all the rubber and flexi hoses are replaced, as I will still have remnants of the old brake fluid in the steel pipes.

THanks for the input.

By the way, anyone know how much fluid the P5B system takes filling from empty, as I need to buy enough to fill it twice (when I fit the new copper pipes once I've found them). 

[Phil Nottingham]

2 Litres? Best to buy 5 Litre can

[lagain]

Bleeding brakes can be a nightmare.
One of our local members did it recently by filling the reservoir, opening the bleed screws and going for a coffee. Apparently the gravity method works well.

[enigmas]

Unfortunately the process of bleeding brakes is a simple technical procedure not clearly understood by many novice mechanics. One fundamental consideration when bleeding a brake system is to be aware that air always rises to high points in any brake system.

Tools & Process
Simple procedures can limit any stress associated with bleeding a system. No sophisticated tools are required. A length of clear PVC tubing to fit over the bleed screws and a clear jar with an inch or so of brake fluid within it. Poke 2 holes in the lid (one for the tube that is immersed in the fluid and a smaller hole to allow air exit.

Bleed the master cylinder at the exit port first. Using your thumb over the exit port of the master cylinder is all that is needed to ensure that it becomes functional as a pump. Once the master cylinder is functionally enabled, the rest of the system can be evacuated of air in short segments if required.

There is no need to rely on bleed points at the extremes of the system. Removing or loosening a brake pipe at a union or component prior to any bleed screw whilst an assistant works the brake pedal in concert with this procedure will readily evacuate air from any point within a brake system. Simply opening and closing a union will often work but if this fails remove the pipe end from the union and place your thumb over the end of the pipe whilst your assistant smoothly pumps the brake pedal. This method will allow the bleeding of controllable segments of the brake system.

Don't forget to check the level of the master cylinder as you proceed.
Never re-use brake fluid that has been pumped through the a system as it is heavily aerated. Doing so is false economy and will only compromise the bleeding process and cause more aggravation

[3litrekiwi]

I had to change the fluid in my Beemer and rather than using the high priced genuine BMW fluid, Bosch ENV6 seemed like a good alternative as the system is a Bosch ABS. It is now in the clutch of the Rover too. Backward compatable with DOT 3, 4 and 5.1 and three year change intervals. 
Cheers
Martin

[ricky8664]

Finally got the brakes finished and bled using DOT 4 fluid as it was a third of the price of silicon. Brake pedal very firm. Not yet run the engine so cannot yet be certain the new servo (sitting on a shelf in my workshop for 30 years) is actually working.

My mechanic had a different way of bleeding the system than I have ever used and we only used 1 litre of fluid to fill/bleed the system.

When I used to bleed the brakes I would keep the reservoir topped up then open the bleed nipple (starting on the wheel furthest away from the reservoir and keep pumping until bubble free fluid would emerge.

His method is to keep the nipple closed, pump the pedal several times until the pedal gets hard, then open the nipple, which causes a flood of bubbles to exit. Then pump the pedal a couple more time to ensure all the air is out, tighten the nipple and move to the next wheel and do the same. Pumping this way with the nipple closed at first seems to move the air towards the wheel being bled under pressure and means you're not pumping the pedal endlessly trying to get the air out, as I used to.

Obviously won't know how well this has worked till the car is road tested after respray and re-assembly but, as I said, the pedal is rock hard. Although I said 1 litre of fluid was used, it's actually less than a litre as there's still a bit left in the 1 litre bottle.

Be interested to see if anyone else has bled their brakes this way (or if I've been doing it wrong all these years!)

[ricky8664]

Thought I needed to clarify that my mechanic's method still involved bleeding each wheel, starting with the one furthest from the reservoir and working forward. Did not mean to imply that only the wheel furthest from the reservoir was bled.

Also, we did not re-use the fluid already bled into the jar, in case anyone thinks that is why we used less than a litre!

[Phil Nottingham]

Oct 31, 2023 10:48:46 GMT ricky8664 said:

Thought I needed to clarify that my mechanic's method still involved bleeding each wheel, starting with the one furthest from the reservoir and working forward. Did not mean to imply that only the wheel furthest from the reservoir was bled.

Also, we did not re-use the fluid already bled into the jar, in case anyone thinks that is why we used less than a litre!

Have heard about that but not tried it as I rarely have the terrible trouble bleeding brakes that some do. I only wait until fluid emerges close and try the brake and then repeat which I suppose is a hybrid of your mechanic's. I always start with the wheel furthest away and never reuse bled fluid which usually amounts to half a jam jar or so. I use a silicon tube with the end blocked off with a blind rivet and a short slit in the tube near the bottom which acts as a one-way valve

[djm16]

What can make bleeding brakes challenging is small air bubbles sticking in crevices and on internal surfaces. Changing the size of the bubbles rapidly with a power vacuum bleeder, and / or kicking the brake pedal hard helps to dislodge the air bubbles.

Note, that in fully flushing the system (and remove contaminants and water), you can bleed the system dry, run through with meths, then blow dry with compressed air. After I did this with my P4 one time (they are even more notorious to bleed), I refilled with Dot 4, and got a full pedal after first try bleeding each wheel.