Master Brake Cylinder Replacement

[Trickydicky]

Hi everyone,

I have to replace the Master Brake Cylinder as the old one is leaking, the question is, I will be refilling with new fluid
(Dot 4) and mixing it with the old fluid already in the wheel cylinders.

Will this have any adverse effects mixing old fluid with new? I dont want to find that I have ruined the seals in the existing brake cylinders.

Anyone have any tips to make the job eaisier?

[minor65]

If your are replacing the master cylinder, then you will have to bleed the complete system / each wheel. So any old fluid will be pushed out when bleeding. Keep topping up with new fluid.

[micstersmoggy]

use silicone brake fluid it doesn't absorb moisture so cylinders don't seize but don't mix with ordinary brake fluid

[bmcecosse]

Oh dear -here we go again I fear! DO NOT use silicon fluid unless you are willing to renew ALL the rubber parts (so all seals and flexis) on the whole system. It offers no real advantages over a sensibly maintained DOT4 system - I just bleed a bit of fluid out of each bleed nipple when doing my annual pre MOT checks. No modern car uses silicon fluid - there is a good reason. It is slightly compressible, and gives a spongy pedal. It's also hideously expensive !

[les]

Come on Roy! Lets have a balanced view, silicon won't strip paintwork or absorb water.

[bmcecosse]

Perfectly true - but then I don't spill brake fluid on my paintwork - and if the cap is kept on the reservoir, and some fluid is bled through once a year - and topped up with fresh fluid from a sealed can - air contact is hardly a huge problem.

[les]

Fair point Roy, my trouble is when I take the tin from the shelf, I unscrew the cap on the way to the car and usually trip over something when approaching, the fluid then shoots all over the car, stripping the paint, then the cap gets stepped on and not replaced. Since changing to silicon it's saved several resprays and I don't need a cap!!!

[bmcecosse]

Hahahahaha - sounds a bit like Mr Pastry of old.........

[IslipMinor]

AP(Automotive Products) who are the original manufacturers of Lockheed braking systems and Borg & Beck clutches will have nothing to do with silicone fluid and advise everyone else to do the same - I would think there could be a very good reason for that!

I would rather have a good solid brake pedal than the theoretical benefit of no paint stripping and non-hygroscopic fluid properties. Brake fluid needs changing regularly anyway, and flushing through every 2 years or so is not a very difficult thing to do, is it?

[StaffsMoggie]

Just try finding silicone fluid for sale anywhere nearby! it is prohibitively expensive when it can be found and offers no significant advantage over conventional fluid when used in a car which is in regular use and has regular maintenance and fluid changes.

[kennatt]

leave the top on untill you need to pour it into the master then put it back on untill you need to top it up simples

[IslipMinor]

TrickyDicky,

There are 2 master cylinder bore sizes used in the Minor. 7/8" for 948 engined cars with 7" front brakes, plus they also used the 7/8" rear brake cylinders as well. The 1098 cars, with 8" front brakes use a 13/16" bore master cylinder and 3/4" rear cylinders.

Which engined car do you have?

[Trickydicky]

I have the 948cc engine minor, it is a 1961 4 door Saloon. I bought the Master Cylinder from Bull Motif and it said in the description on the web site that it fits all models.
I have read that it does not matter which M/C you fit, is this correct? I was hoping to get it fitted this Thursday as I have some time off before Xmas.
I did ponder the idea of just replacing the the seals but as you have to take out the Master Cylinder anyway it seems like a no brainer.
When I take out the old cylinder I will check the bore to look for any signs of scratching and if not will add it to the forsale board for the price of the postage.

[rayofleamington]

AP(Automotive Products) who are the original manufacturers of Lockheed braking systems and Borg & Beck clutches will have nothing to do with silicone fluid and advise everyone else to do the same

Certainly as none of their brake-fluid compatible parts were validated with silicon fluid that is exactly what they should say!
Automotive hydraulics seals are validated with the vehicle manufacturer's specific brake fluid blend and in some cases also with standard off the shelf fluid.

As for AP having nothing to do with silicon fluid... not 100% true - one of their divisions used Castrol TLX (a blend of DOT 4 and silicon fluid) in their mass production transmission actuators.
Like most of AP - that division was sold off to help keep them afloat before they sold their trading names & logos (to delphi distribution) and a chinese firm bought out their UK manufacturing base.

[bmcecosse]

Aaahhh soh...

[IslipMinor]

I have read that it does not matter which M/C you fit, is this correct?

I would fit the later smaller bore one as a preference to all models, but definitely not the early 7/8" to the later 1098 system - the pedal pressure will be too high because of the small rear cylinders. The smaller m/c with early 7/8" rear cylinders will cause a slightly 'longer' pedal, i.e. more travel, but should still have a good firm feel.

[Trickydicky]

oops too late now as I have bought the one from Bull Motif
http://www.morrisminorspares.net/shop_item.php?ID=1489

It does not state what size the M/C bore is on the one advertised just that it is not a pattern part.It must be stepped down further down the cylinder as measuring the front where the circlip is fitted is 27mm in dia.At a guess I would say it will be 13/16" bore.
I will keep an eye on the brake cylinders for signs of leakage after I have fitted it.

[les]

It's all been said before, we all defend our choices! I've used silicon for 10years, it's accepted it won't absorb water, so doesn't need changing. Prior to this I used dot 4 and replaced seized wheel cylinders every so often!

[rayofleamington]

There is no perfect solution for all classic owners.
For those who use their cars infrequently there is an increased risk of seized cylinders - regular use helps keep the cylinders dry (they get hot in use). Wet DOT4 helps the corrosion process... Silicon fluid would be a benefit but is more costly, should not be used in combination with old seals previously used in brake fluid and can cause trouble to get a firm pedal.. Mine bled out fine with silicon and pedal was firm on a complete new braking system.

[Trickydicky]

Hi everyone, hope you all enjoyed the festivities

I have changed the Master Cylinder, the one I took out had 13/16" cast on the side so at least I know the size....
Removal was straightforward following the instructions in the Tech Tips section and also with the Haynes Manual to hand. The hardest job was refitting the brake pedal return spring (my hands are too big for the small gap behind the brake pedal and did not have a pair of long nose pliers big enough) cue profanitys
I now have the job of bleeding the brakes, following the method in the haynes manual I dont seem to be able to get rid of the airlock in the new cylinder. I have the wife helping and she is getting frustrated at the speed of the operation....
Bmcecosse, you mentioned in another thread allowing gravity to bleed the brakes, could I just keep the brake pedal depressed, open the bleed screw and wait for the fluid to start appearing at the rear cylinders first? (obviously I have to keep an eye on the fluid level in the cylinder then repeat for the other wheels? As the car is up on axle stands the front is higher than the back at the moment)
or is the wife being too impatient and should I carry on bleeding the brakes as normal?

Many Thanks and here is a picture of the other woman in my life

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