Thread: Brake Master Cylinder

If there is no loss of fluid, you are right to suspect the master cylinder - assuming it was not brake fade, which you are probably unfamiliar with. If the hill was steep enough, and long enough, the front (mainly front) brakes heat up, expanding the drum, which means that the brake shoes have to move further to contact the drum - hence the increased pedal movement.

Since the hydraulic leverage is very large, since you don't have a booster, the amount of expansion needed to cause this effect is not very much.

John

Yep agree. Master cyl bypassing internally. Can be caused by a little bit of contamination that may have passed out of the master cyl and into the lines. Give it a bleed to replace all the fluid. If it happens again then put a kit through the master cyl.
Hope this helps.

Cheers Rod

Sent from my GT-I9507 using AULRO mobile app

Thanks Gents. No, not heard of Brake Fade. I'll keep my eye on it.
cheers,
D

Originally Posted by Dark61

Thanks Gents. No, not heard of Brake Fade. I'll keep my eye on it.
cheers,
D

Brake fade was something all drivers knew about before discs became almost ubiquitous. One of the major advantages of discs is that when the discs get hot the pedal does not recede. Disc brakes can fade, but since this does not happen until the fluid boils, it takes really severe mistreatment for it to happen.

(The other major advantage of discs is that they are pretty much immune to failing because of water acting as a lubricant between the shoe and the disc, because it is thrown off by centrifugal acceleration, which on a drum holds it on the friction surface. Important on an offroad vehicle.)

John

OK. So I took the front passenger side wheel off and its as dry as a bone inside - no leaks. The wheel spins with some resistance. Just jacked the drivers side up and similar resistance. Both rear wheels spin much more freely. Is this par for the course , or does this suggest the rear brakes need adjusting?
cheers,
D

Originally Posted by JDNSW

Brake fade was something all drivers knew about before discs became almost ubiquitous. One of the major advantages of discs is that when the discs get hot the pedal does not recede. Disc brakes can fade, but since this does not happen until the fluid boils, it takes really severe mistreatment for it to happen.

It is true that modern brakes are more resistant to fade, but it still happens. The brakes on my Hilux faded almost completely when they overheated coming down the long hill in to Sofala with a car trailer that (I discovered) had a failed electric brake controller.

Brake fluid absorbs water, so old brake fluid boils at a lower temperature. When the pedal is released the steam expands, and the next time the pedal is depressed the lines have gas (steam) in them, so creating excessive pedal movement exactly like brakes that need bleeding. Brake fade can also be caused by softening of hoses due to heat and, as JD describes, due to component expansion. These are sometimes referred to as "pedal fade".

The classic cause of brake fade is overheating of the pads or shoes, which then start to release gases that reduce the friction between the pad/shoe and disc/drum. The pedal still feels hard, but the braking effect is reduced. For racing, there are much "harder" grades of friction material that run at much higher temperatures but don't work well until they heat up. I once sold a street legal track car to a guy, who later came back and apologised that he had been so dismissive of my warnings about cold brakes. Apparently he rather scared himself when he first applied the brakes, and was all set to bring the car back until he remembered what I had said and let them get up to operating temperature!

If the brake fluid in the OP's vehicle is old and has a high water content, and especially if the brakes are dragging, it is possible that the fluid boiled. That would explain the temporary "lack of pedal" [Edit; but less likely if the pedal went to the floor "slowly"]. However any kind of temperature related fade is normally accompanied by a rather unpleasant burning smell and sometimes smoke.......

Originally Posted by Dark61

OK. So I took the front passenger side wheel off and its as dry as a bone inside - no leaks. The wheel spins with some resistance. Just jacked the drivers side up and similar resistance. Both rear wheels spin much more freely. Is this par for the course , or does this suggest the rear brakes need adjusting?
cheers,
D

I figure that if you've got a wheel jacked up you might as well check/adjust them anyway, it only takes a minute.

Something else that I have discovered is that the pin on which the adjuster cam pushes is not quite correct on some brake shoes. I had a set of nearly new shoes (no idea of the make, they were on a vehicle) where the pin was sufficiently "wrong" that the snail cam would slip passed the pin and rotate 360degrees. I assumed the cam was worn or broken, or possibly the drum and shoes were badly worn, but when I took the drum off it became clear that the cam was fine and the shoe was at fault. It was possible to adjust the brakes, but I'm not convinced that they would stay adjusted! Comparing those shoes to another set (Mintex) showed the pins were much shorter and in a different place........

Regarding master cylinders, which version do you have? The normal dual circuit for '74 would be the 90577520 which has the reservoir attached with two vertical screws, but it could also be single circuit or possibly the later (supposedly 1980 onwards) NRC6096 dual circuit with the reservoir attached with two roll pins.

I have tended to use TRW (Lucas) cylinders, but I have recently bought some Delphi ones as they are significantly cheaper and seem to be OK. The reality is that these companies have been bought and sold many times. TRW bought Lucas, then split it up and sold most of it and I was told the automotive brakes portion was later sold to ZF, yet we can still buy TRW labelled brake components so who knows? I have also read some reports of poor quality parts from TRW, but I have never had any problems.

I stick to TRW/Lucas or Delphi for cylinders, and Mintex for shoes. I have found Britpart, Bearmach etc. sometimes sell good quality parts, occasionally they even supply genuine LR parts repackaged under their own labels, but just as frequently they sell poor quality rubbish. I was given a set of Britpart shoes some time ago, and whilst I cannot say what they were like in use because I never fitted them, the friction material seemed (under the exacting "thumbnail" test!) to be noticeably harder than the Mintex version. I'd rather not take the chance with brakes, so I stick to branded/OEM products.

As, by coincidence, I'm also working on 109" brakes this morning I thought I'd elaborate on the adjustment issue I mentioned above. Forgive me if this is all common knowledge, but I'd not seen it before.

The brakes are adjusted by turning a snail cam which pushes the shoe outwards until it contacts the drum and interferes with its rotation, and then backing off a fraction until the wheel turns "freely". The snail cam pushes on a steel "pin" on the shoe.

On the Mintex shoes the pin protrudes 16mm on each side of the shoe. On the "problem" shoes, the pin protrudes only 8 or 9mm.

When the brakes are adjusted, the snail cam should push on the pin. Unfortunately if the pins are too short to correctly engage with the cam, the cam has a tendency to simply slip underneath the pin and keep rotating. At some point, presumably when these shoes were fitted, the "skilled mechanic" doing the job discovered this issue. The "fix" was to back off the steady post, allowing the shoe to move towards the back plate, providing better engagement between the snail cam and the pin. This does, when stationary, appear to allow the brake to be "adjusted". What actually happens is that as the cam is turned the shoe moves outward, but also straightens itself back to where it would be if the steady post was correctly adjusted. IF the outside edge of the shoe contacts the drum before the shoe has hinged far enough for the snail cam to slip under the pin, then the shoe contacts the drum and APPEARS to be correctly adjusted. Applying sufficient "spanner" to the adjuster will cause the shoe to continue to hinge, and therefore the cam will slip under the pin and continue to rotate, but because this cycle is repeated every 360degree rotation of the adjuster, the "skilled mechanic" can stop when the edge of the shoe just kisses the drum, and fool himself that the brakes are adjusted.

In use, however, the shoe straightens itself when the brake is applied (which is actually the procedure for setting the steady posts) and when the brake is released it moves both backwards as the cylinder retracts and inwards until it contacts the steady post. It is now a lottery as to whether the snail cam catches the pin in the correct manner, or the pin rests on top of the snail cam which then becomes, in effect, the steady post. If the pin rests on the top of the cam, there is nothing to stop it continuing to be pulled back by its return spring, until the wheel cylinder piston bottoms out.

Because the shoe straightens, the wheel cylinder has to push it further than would be the case for a genuinely well adjusted system so the pedal inevitably has to travel further, but potentially not so much further as to be a concern. Because the cam may sometimes catch the pin correctly but other times slip underneath, pedal travel may be variable. As the shoe wears, there comes a point at which it's edge will no longer touch the drum before the cam slips underneath the pin and at this time the "skilled mechanic" attempting to adjust the system has to decide whether to ignore the excess pedal travel, or investigate.....

On the vehicle in question, the pin length and cam positions were such that after very little usage (the shoes still looked new) one shoe would not adjust at all as the cam slipped underneath the pin before the shoe touched the drum, one could be "adjusted" as described above though fractionally too much spanner resulted in a full rotation, and the remaining 2 front shoes had sufficient pin/cam engagement to actually work correctly.

Assuming, as was the situation with this vehicle, that the adjustment cams are in good condition and not bent, loose or worn out, the Mintex shoes with 16mm adjuster pins work perfectly and are long enough to cater for a degree of variation in the distance the cams stand out from the back plate. That is not the case with the shorter pins.