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[Abiton]

I can't go any more mental than I already am Webby, I've been mad for years...

Convection currents occur in fluids when there are temperature differentials, warm fluid rising (because of reduced density) and being replaced by cooler fluid that falls round the outside of the rising central column.  I can't see any reason why this wouldn't happen within automotive brake systems, as the hottest bit is also the lowest bit.

I may be wrong, but basic physics says I might be right. 

Simply put, the pipe bores are too small. For convection currents to work there has to be room for the fluid to actually move in two distinct "cylinders" .. the inner one is the hot fluid rising, around the outside is a hollow cylinder of cold fluid falling ... has to do that or you end up with a vaccum at the bottom !! pipe bore of a brake pipe is not wide enough to set that up IMHO, so the fluid in the caliper ends up getting hot then cold ... lots.. Also, fluid does not "move" as such in the brake system , it is compressed and moves the piston a miniscule amount each time, the only real "movement" being the replacing of pad wear ...  so well worth replacing that small amount of fluid every time.

I don't see that, brake fluid is pretty damn runny, even at low temps.

Someone on the www must have researched this. Got an unbusy morning tomorrow, I may just have a go at finding some info. 

[Webby the Bear]

I can't go any more mental than I already am Webby, I've been mad for years...

Convection currents occur in fluids when there are temperature differentials, warm fluid rising (because of reduced density) and being replaced by cooler fluid that falls round the outside of the rising central column.  I can't see any reason why this wouldn't happen within automotive brake systems, as the hottest bit is also the lowest bit.

I may be wrong, but basic physics says I might be right. 

Simply put, the pipe bores are too small. For convection currents to work there has to be room for the fluid to actually move in two distinct "cylinders" .. the inner one is the hot fluid rising, around the outside is a hollow cylinder of cold fluid falling ... has to do that or you end up with a vaccum at the bottom !! pipe bore of a brake pipe is not wide enough to set that up IMHO, so the fluid in the caliper ends up getting hot then cold ... lots.. Also, fluid does not "move" as such in the brake system , it is compressed and moves the piston a miniscule amount each time, the only real "movement" being the replacing of pad wear ...  so well worth replacing that small amount of fluid every time.

I don't see that, brake fluid is pretty damn runny, even at low temps.

Someone on the www must have researched this. Got an unbusy morning tomorrow, I may just have a go at finding some info. 

nooooooooooo. dont do it abiton. i dont want to see the common heading: ''another brake fluid thread'' 

[Toledodude1973]

On a more my level of discussion do you clamp the brake hose with mole grips or something similar?

[Field Marshal Dr. Opti]

I had to reuse the old pins. The ones that came with the pads were too short.

[Entwood]

On a more my level of discussion do you clamp the brake hose with mole grips or something similar?

I use mole grips ..

and large plumbers pliers to ease the pistons back ... there's a flat on the caliper and other the large jaw of the plumbers pliers sits nicely right across the piston ensuring it stays flat ... gentle squeeze and it eases in nicely

[Abiton]

Webby: 

I think it's an interesting subject.  You must have studied master cylinders and their general arrangement, so you may be able to see that as long as pistons are pushed back rather gently/slowly there is no risk of damaging seals; so that only leaves the contention that there is this 'dead spot' of fluid that never moves from the piston bore.  The latter seems pretty unlikely IMHO.

Edit: Diffusion, irrespective of convection, will tend to annul any gradients of pretty much anything that might vary within a fluid anyway, no?  (Imagine a fart spreading around a room, or a wee in a swimming pool.  )

[Webby the Bear]

Webby: 

I think it's an interesting subject.  You must have studied master cylinders and their general arrangement, so you may be able to see that as long as pistons are pushed back rather gently/slowly there is no risk of damaging seals; so that only leaves the contention that there is this 'dead spot' of fluid that never moves from the piston bore.  The latter seems pretty unlikely IMHO.

i'll be honest mate we havent looked at brakes at college yet (next few weeks i think). everything ive learned has been either from here or eric the car guy on youtube! 

i'm going to bale out of the ''is the fluid bad behind the piston'' cos i dont know. i'm only going on what i've learned and been told. 

however this is exactly what i use.....



vice grips with two pieces of fuel line to protect brake hose 

[TheBoy]

If your just changing the pads them there is no need to open the blead nipple to push the piston back, and unless your planning to bleed the rear brakes then leave well alone,

You increase the risk of seal damage or inversion in the master cylinder.

[TheBoy]

Surely convection currents within the fluid will ensure that it is mixed to a good extent?  In other words, there simply won't be any localised concentration of 'bad' fluid near the piston. 

It doesn't. You get the shagged (really shagged if its very dark) fluid stuck in the caliper. Its a hydraulic fluid in reality, its not going to mix much, it all stays pretty static.

Project TB2 - Tractor! - had brakes that were prone to going spongy when I got it. The fluid clearly wasn't that old, but clamping the hose, and pushing the front pistons back in, it was almost black. The rears were better, but noticibly darker.

Sponginess cured.

[TheBoy]

Actually, it was more than spongyness, it was the pedal going to the floor after half a dozen roundabouts

[Abiton]

Surely convection currents within the fluid will ensure that it is mixed to a good extent?  In other words, there simply won't be any localised concentration of 'bad' fluid near the piston. 

It doesn't. You get the shagged (really shagged if its very dark) fluid stuck in the caliper. Its a hydraulic fluid in reality, its not going to mix much, it all stays pretty static.

Project TB2 - Tractor! - had brakes that were prone to going spongy when I got it. The fluid clearly wasn't that old, but clamping the hose, and pushing the front pistons back in, it was almost black. The rears were better, but noticibly darker.

Sponginess cured.

OK, that's useful info.  Your reported driving style would also tend towards 'maximising the chance of convective mixing', I think?

I don't get why a hydraulic fluid won't mix within itself though.  It's just a fluid.  Try peeing in the bath and keeping the yellow patch distinct from the rest. 

[Andy B]

.... so you may be able to see that as long as pistons are pushed back rather gently/slowly there is no risk of damaging seals;  ....

I hope your hair shirt is laundered & ready to wear!!! 



I push the pistons back & don't usually open the bleed nipple either! 

[Andy B]

I had to reuse the old pins. The ones that came with the pads were too short.

because not everyone knows that later 2/6 & 3.2 cars have vented rear discs ..... the short pins are long enough for the older solid discs 

[Webby the Bear]

I had to reuse the old pins. The ones that came with the pads were too short.

because not everyone knows that later 2/6 & 3.2 cars have vented rear discs ..... the short pins are long enough for the older solid discs 

Eurocarparts sell two different pins.... tell them if youve got vented or non-vented and theyll supply the correct pins seperately 

[Andy B]

I had to reuse the old pins. The ones that came with the pads were too short.

because not everyone knows that later 2/6 & 3.2 cars have vented rear discs ..... the short pins are long enough for the older solid discs 

Eurocarparts sell two different pins.... tell them if youve got vented or non-vented and theyll supply the correct pins seperately 

but there are also some garage mechs that have fitted solid discs to the back of a car that should have vented ...... all OK to start with until the pads wear & one of the pistons makes a bid for freedom from the caliper. Result is brake fluid everywhere & no brakes.