11 replies to this topic

[Tompat3463]

as the title really im building a front engined fireblade mini and im nearly ready for a test drive as soon as I fit all the braking gear but im wondering what the brake compensator does and if there is any upgrade for it instead of putting the old crappy style thing back in.

 

thankls Alan

[HarrysMini]

It's a brake limiter valve. It limits the amount of fluid that goes to the rear brakes.

 

Someone will be along in a minute with more information. 

[Captain Mainwaring]

It's a brake limiter valve. It limits the amount of fluid that goes to the rear brakes.

 

Someone will be along in a minute with more information. 

 

 

Limits the pressure to the rear brakes - doesn't really limit the amount of fluid.

 

You could fit an adjustable compensator then you can balance the brakes to suit the trim of the car.

[HarrysMini]

 

It's a brake limiter valve. It limits the amount of fluid that goes to the rear brakes.

 

Someone will be along in a minute with more information. 

 

 

Limits the pressure to the rear brakes - doesn't really limit the amount of fluid.

 

You could fit an adjustable compensator then you can balance the brakes to suit the trim of the car.

 

I knew I had something wrong, thanks for clearing that up.

[tiger99]

If only it were that simple..... And yes, an adjustable pressure limiter would be useful, provided it was designed so that the rear brakes had full pressure available to them in the event that there was a failure in the front circuit, as the bulkead-mounted PRV is supposed to achieve.

 

Actually, I often wonder why people who fit Metro twin pot calipers, a decent enough modification, don't go all the way and plumb them for dual circuits at the front, which would somewhat relax the problem of what to do with the rears if the fronts fail.

 

There may be legal issues with adjustable balance valves (strictly impossible anyway in a hydrostatic system, they are really just limiters). although I am not sure that we have fuly understood the legal ramifications yet.

 

By the way, a brake compensator is something else altogether, and is effectively a balance bar that equalises the handbrake force on both rear wheels. It is a ball-jointed bar passing through the handbrake lever on early models, into which both cables are screwed, with adjusting nuts. On later models, it is under the car, suspended between front and rear cables.

[Ethel]

http://www.rallydesi...oducts_id=15436

[tiger99]

I see that Willwood do not show proper performance details for that particular valve.If they did, it would reveal that, like all such valves, it is, at best, a soft limiter. A true proportioning valve is physically impossible in a hydrostatic system, without using what would effectively be a slave cylinder driving a master cylinder via a variable geometry linkage. But then the necessary bypass to allow the system to be bled would completely mess up the proportioning action.

 

Unfortunately true proportioning is what is really needed, and no valve ever made has, or will, achieve that in a hydrostatic system. A balance bar at the pedal can do exactly what is required, except that you are apparently not allowed to make it adjustable, at least if your car requires an IVA test. The other answer is of course to go for a "power", i.e. pump-driven, hydraulic system, Citroen style, which makes lots of other interesting things possible too. Efficient, fast-acting, properly proportional ABS for one. Also, true load-sensing brake pressure proportioning, and constant ride height suspension. And yes, the IVA does seem to allow that, although very few car modifiers would have the necessary knowledge to fit such a system safely in a strange car, there being many pitfalls.

[tiger99]

Here is a good, honest article about the subject.

 

http://www.stoptech....rtioning-valves

[Captain Mainwaring]

If only it were that simple..... And yes, an adjustable pressure limiter would be useful, provided it was designed so that the rear brakes had full pressure available to them in the event that there was a failure in the front circuit, as the bulkead-mounted PRV is supposed to achieve.

 

Actually, I often wonder why people who fit Metro twin pot calipers, a decent enough modification, don't go all the way and plumb them for dual circuits at the front, which would somewhat relax the problem of what to do with the rears if the fronts fail.

 

There may be legal issues with adjustable balance valves (strictly impossible anyway in a hydrostatic system, they are really just limiters). although I am not sure that we have fuly understood the legal ramifications yet.

 

By the way, a brake compensator is something else altogether, and is effectively a balance bar that equalises the handbrake force on both rear wheels. It is a ball-jointed bar passing through the handbrake lever on early models, into which both cables are screwed, with adjusting nuts. On later models, it is under the car, suspended between front and rear cables.

 

 

I have heard in the past the term compensator applied to load proportioning valves, and yes, as you say, it is equally and correctly applied to the handbrake cable balance bar.

 

I did once try to explain a little regarding hydrostatic braking systems - it fell on deaf ears, however, the long and short of it is that in a perfect braking system, there will be little or no flow. Flow is required to allow feedback and thus pressure regulation.

[Earwax]

An area i am totally ignorant of, but wanting to learn more.  If someone could please assist with the allround knowledge.  RE: drum front and rear... what limiter/ disc fronts are limiters all the same across variants eg 7 inch, 7.5 8.4 .

 

Are there different settings available, ( not adjustable) but swap out ( like different temp rated thermostats) and finally in single circuit vs dual... i take it limiters would be different?? how

[tiger99]

Well, basically the limiter is only in the rear brake circuit on any Mini, and a few don't use one at all because they have very small rear wheel cylinders to get correct balance. These were mostly those with a diagonal split system. There are two main types of pressure limiter, well actually 2.5, the original looks like a big, fat T piece, and lives in a hole in the rear subframe front cross member, where the one brake line from the front splits to go to each rear brake. That is only used on single circuit brakes.

 

Twin circuit brakes require that if the front circuit fails, the driver is given the best possible chance of stopping with what remains. (A very poor option, so always maintain your brakes such that a failure is not going to happen!) That means that a bulkhead mounted PRV (pressure reducing valve, strictly just a fixed limiter) is used, which uses the pressure in the FRONT circuit to close the valve to the rears when it rises high enough, so if the fronts fail, pressure passes through directly to the rears.

 

The space between the various bits of the PRV in the front circuit and the rear circuit is vented to the outside air, so that there is no possibility of an internal seal failure causing an undetectable leak from one circuit into the other. The space between the front and rear seals in the master cylinder is also vented, back to the reservoir, so again if a leak is developing between circuits, it is indicated to the driver by the pedal sinking slowly to the floor. A failure introducing a leak from one circuit to the other, which was not detected, would be classed by safety professionals as a "dormant" failure, and is a horrendously dangerous thing because it ensures that when the slightest pin-prick leakage develops in one circuit, both fail together. I have seen such a failure on an old and rusty Peugeot, with an incompetently designed ATE master cylinder, but I think that the major manufacturers have taken care of that sort of thing ages ago.

 

Watch out for Willwood and other aftermarket parts, or anything where both circuits pass through one body, the necessary safety precautions may not have been put in place.

 

Now I said 2.5 types of pressure limiter, the odd 0.5 is really just a bulkhead mounted PRV with a switch added to operate a warning light if there is an improper pressure difference between the two systems. It is called a PDWA (pressure differential warning actuator), a very bad bit of terminology.

 

Coming back to dormant failures, to ensure detection of a loss of pressure in one of the two independent sysems, the PDWA was intdoduced, but there was also a type built into the master cylinder itself, hopefully now obsolete, as it itself added undesirable failure modes, and was often rebuilt incorrectly. For some reason, a pen-pushing idiot somewhere, possibly the same one who had introduced regulations to introduce the PDWA (common on many cars of that era), then changed his or her mind, and changed the rules so that detection of low fluid level was sufficient to indicate failure of one system. It isn't, as it activates far too late to prevent some accidents, and comes with a "brake test" switch that is not understood by most people, far less used, which only tests the bulb, not the switch on the master cylinder, the part that usually fails. So, a PDWA is probably a good idea, but I would advise against arbitrarily reconfiguring the brake system of any Mini, except by putting it all in one of the standard configurations that are known to be correct, not a hotch-potch of odd bits.

 

Some vehicles have a load sensing limiter valve in the rear brakes, this being a limiter with its spring force controlled by a link to the axle

so that as the back end sits lower when loaded, the valve lets more pressure through to enable extra braking to be provided without wheel locking. They were more or less essential on pickup trucks at one time, but are now out of fashion. They were unreliable, and accidents resulted from them leaking due to corrosion around the actuating plunger, or from massive wheel locking due to them siezing up in the heavily loaded position, the load subsequently being removed. They also have a very serious weakness, because if the brakes are applied as the rear wheel(s) hit a bump, the brake pressure limit will be set according to the raised position of the axle, and the high pressure will allow wheel locking as soon as the bump is passed.

 

As far as I know, there have been adjustable PRVs and limiters, with a screw in one end to set the spring force, but I don't think they are available now, and I would not trust any hydraulic component that had not been made by a major manufacturer such as Lockheed or Girling, so I would not want an aftermarket part, made in China, in my brake system. If the cast body of the PRV turns out to be made of Chinese cheese, and bursts, you have no brakes at all, except the fairly inefficient handbrake.... Don't know about Willwood, I suspect that they are well made, but there seems to be even less control of aftermarket manufacturers in the US than in the UK, with a lot of stuff sold to the hot rodders that would be illegal here.

 

As for earlier mention of hydrostatics, the flow rate in a car brake system, even when you jab the pedal hard, is utterly negligible, and there is no significant pressure drop in the brake lines (except that some rubber flexibles allegedly collapse internally, causing blockage, something I have never seen in any of my vehicles). The fluid is essentially incompressible, but there is a little bit of stretch in hoses, and a small amount of fluid is displaced in pushing the pistons from their rest positions, until the pads or shoes are in firm contact, so for the pedants some small amount of fluid does indeed move backwards and forwards in the system, but the effect of that on pressure drop is not worth calculating. The pressure limiting valve, of whatever type, operates by blocking flow towards the slave cylinder above a certain pressure.

 

Edit: See below for the various brake system configurations used on Minis:

 

http://www.somerford...page=page&id=57

http://www.somerford...page=page&id=58

http://www.somerford...page=page&id=59

http://www.somerford...page=page&id=60

http://www.somerford...page=page&id=61

http://www.somerford...page=page&id=62

http://www.somerford...page=page&id=63

http://www.somerford...page=page&id=64

http://www.somerford...page=page&id=65

http://www.somerford...page=page&id=66

http://www.somerford...page=page&id=67

 

 

Also, it is a safety and legal requirement that the master cylinder can always displace sufficient volume of fluid to take up all slack between shoes and drums, or between pads and discs, allowing for maximum wheel bearing slop, disc wobble, etc, in either brake circuit if the other circuit has suffered a complete failure. That means that in many cases, brake modifications need a larger diameter master cylinder, with the consequence of a heavier pedal. I suspect that many people (not necessarily Mini owners) have done modifications without the necessary tests and/or calculations, and are driving around with brake systems that are fundamentally unsafe.

Edited by tiger99, 28 February 2014 - 05:36 PM.

[Earwax]

excellent post, tiger  thank you for such detail. improves my understanding no end