More GT6 rear brakes!

[mikeyb]

Having found one dodgy pipe flare when removing a rear driveshaft from my Mk1 GT6, for piece of mind I want to drain the system down and check the rest of the remade copper pipes inherited with the car. Whilst doing this I intend to change all the brake seals and go back to DOT4 brake fluid from the silicone DOT5 that it currently has.

Whilst browsing traders for seal kits etc. it became apparant that Mk1 rear wheel cylinder piston diameters changed from 3/4" bore to 5/8" bore at chassis number KC7279.

So the big question is WHY?  

I cannot see any changes to other brake components associated with this and would assume that the change would have been to facilitate some sort of improvement. My car has the earlier size so does anyone know what effect fitting the later smaller cylinders would have?

Cheers

Mikey

[Richard B]

Driving in a 'spirited' fashion in a GT6 would cause the rear to lock up more easily with the larger bore, due to weight transferrance under braking.

Presuambly the car would be more tail happy that way?  ::)

[thescrapman]

I do despair at times, it does seem you didn't pay attention in Physics when we were at school.. :-)

Richard clearly did pay attention when he was studying Physics...

It might be worth trying the smaller cylinders, (Spitfire size?) if you are going to swap them over anyway.

Cheers

Colin

[mikeyb]

I was going to just change the seals, but with a seal kit costing £3 vs a complete new cylinder for £6, it seems hardly worth messing around.

Physics?  Did I really do that?    :-/

[Nick Jones]

Where do you work again?  :o :P

Nick

[sam93]

Out of interest,  what's you're reason(s) for going back to dot 4 type fluid ?

[DougBGT6]

The late mk3s slave cylinders increased to 11/16th so, where's the physics in that?  ;D

Also, naïve question, what's wrong with silicon brake fluid?   :-/

[DougBGT6]

Ah..... the changes in diameters coincide with the changes to rear suspension. When they introduced the rotoflex thingy they dropped the diameter, when they went back to the old suspension, (redesigned to work less scarily) they increased the diameter. Or is this all coincidence? Or was it all to do with what they had laying about in the stores at the time?  ;D

[mikeyb]

DOT 4 is less compressable than DOT 5 so it gives a firmer pedal feel.  Also whilst 5 does not absorb moisture like 4, any moisture that does get in (and it will) drops toand collects at the lowest points - ie the wheel cylinders/calipers - therefore needs bleeding more regularly than 4.

But the biggest reason in the cost of the blasted stuff!  With all the cars in my fleet, I need to standardise to keep things simple.  I can by DOT 4 by the gallon cheaply, but not DOT 5!

The well known advantage of DOT 5 of course is that it does not hurt paintwork when spilled, but for me that does not outweigh the above.

Rgds

Mikey

[DougBGT6]

Makes sense. Respec, dude  8)

[Tim Bancroft]

I have issues with silicon brake fluid boiling up at ower temps than mineral based fluid, I would not entertain using silicon fluid again after a hairy trip down a mountain in the Alps-that was exciting!

[Clive]

For me it was the concern if I should need any extra brake fluid. Getting dot4 is simple. Ask for silicone and you are stuffed. So it makes sense to use dot4. Well, for me.

[BrendanD]

I feel the opposite. Keeping a can of Dot 5 in the boot is no problem, it tucks in the side of the rear wing.
I had the fluid changed in the Spit after 9 years, never needed bleeding and no issues apart from a slightly leaking master cylinder at 4 years, just felt it was due a change.
I used it after a complete brake refurb, new lines, cylinders and goodridge hoses. After 9 years and 25,000 miles I just changed the seals and fluid.
The firmer pedal is restored by using the stainless braided hoses which are less extensible.

I can't imagine how many times i would have needed to have had the bulkhead stripped and repainted in that time. £50 of fluid probably paid for itself more than 10 times over...