Soft brake pedal

[C R Box]

We rally a 1963 Austin Healey 3000. The car originally had rear drum brakes.Now its got a rear disc conversion.The problem is that the (acceptable and difficult to alter) axle end float, allows the pads to be pushed away from the rear discs, during cornering.Consequently the pads have to travel a longer distance before they meet the discs, resulting in a disconcertingly soft brake pedal.
Anyone, with an idea for a fix?

[Dave Brand]

Originally posted by C R Box
Anyone, with an idea for a fix?

The only ideas that spring immediately to mind are either floating discs, as used on motorcycles (unobtainable?) or floating calipers, which would mean a lot of re-engineering (and possible homologation problems?).

You could reduce brake pedal travel by using a bigger-bore master cylinder, but that would reduce mechanical advantage in the system, thus increasing pedal pressure.

.....or revert to drum brakes!

[C R Box]

Thanks for replying DB.
In retrospect, possibly the drum brakes should have been retained.
However,on a long distance road rally, with mountain ascents and descents, we will 'do' all 8 pads in two days. I'd expect the front pads to go even quicker, with rear drums.As we do our own servicing, the last thing you want at the end of the day is a time consuming brake change over.
As for homologation issues, obvious non period (type)alterations are a prohibited.
Re-engineering is difficult as it's all extremley tight and compromised already.
Some peaople fit springs inside the pad pistons, but I can't beleive that this is really ideal. I mean, if the springs are strong enough to overcome the seals, then surely they'll cause pad wear.
I was hoping someone may know, of a non return valve, type device, which could be 'discreetly' positioned.

[Dave Brand]

Originally posted by C R Box

In retrospect, possibly the drum brakes should have been retained.
However,on a long distance road rally, with mountain ascents and descents, we will 'do' all 8 pads in two days. I'd expect the front pads to go even quicker, with rear drums.As we do our own servicing, the last thing you want at the end of the day is a time consuming brake change over.

On that basis the rear discs must be the better option. Linings on the rear would probably last several times as long as a set of front pads, but then they would fade faster, giving the front pads more work to do. I'd stick with discs all round!

As for homologation issues, obvious non period (type)alterations are a prohibited.
Re-engineering is difficult as it's all extremley tight and compromised already.

That's a no-no, then!

Some peaople fit springs inside the pad pistons, but I can't beleive that this is really ideal. I mean, if the springs are strong enough to overcome the seals, then surely they'll cause pad wear.

That jogged a little memory down in the dim, dark recesses of my brain! I dug out my Haynes Brake Manual &, sure enough, there's a fixed caliper with piston springs - Chevy Corvette. However, it doesn't use conventional square-section seals. The seals are on the piston & look just like normal drum brake seals, so the springs can be light enough, just like the springs in a drum brake wheel cylinder, to keep the piston extended without applying appreciable pressure to the pads - drum brakes also have much stronger springs to retract the shoes. In addition to wearing the pads I would think a spring strong enough to overcome the seals could cause a lot of heat to be generated; not desirable!

I was hoping someone may know, of a non return valve, type device, which could be 'discreetly' positioned.

I've never heard of such a device - it would have to be engineered to allow a certain volume of reverse flow otherwise it would just lock the brakes on!

[GTV27]

Springs behind the pistons is not uncommon - light pressure on the pads will not cause undue friction or wear, and will reduce the extent to which axle flex or warped discs will cause pad knock off.

If heard of the rear line being pressurized (air pump in the cabin - gives the navigator something to do), but am not sure about its effectiveness.

A one way value would just lock the rear brakes on.

I have a similar problem with my Alfa GTV - but it is not very serious and a couple of left foot prods on a straight after a high g corner using brings the pedal right back up - with a big healey, you'd need all the brakes you can get . Next time the rear calipers are apart, I will be trying some light springs.

[Onlooker]

The only things I can think of are already mentioned as in the piston springs and are/can be used in all forms of racing to prevent knock off.
Floating rotors are going to be quite expensive by the time you buy the hats,rotors and bobbins
Air pressure will also do the trick,and is probally easy to hide or get passed for use.I personally have never done it but I know people who have and reckon that it is very effective.I think that they put a air valve in the m/cyl lid (an air tight one of course) and just pumped it up with a bike pump as it only required a low pressure.You may want to get a little pressure gauge and screw on attachment like used for shocks to acuratly do this job as the results will no doubt be a dependent on a few factors ie 1 to much air and leaks
2 not enough and reduced effect when the pads wear and the volume changes in the resivour.
Best of luck

[norman-normal]

Only two things will cause "soft pedal": air and rubber flexable lines. Sence fluid is not very compressable, and your system is most likey bleed properly, the softness could only come from flexable line expansion 'balooning'.

Install stainless steel braided flexable lines and your "soft" pedal problems are over.....This stuff about the axel, could cause longer pedal travel, not "soft".
You must rember, disk require much higher fluid pressures than drum. A disk master cly will, generaly have a smaller diameter and a longer stroke to generate higher pressures; and is one of the lessor knowen problems of converting drums to disk. "Tilton" makes a dual M/cly set up with cast alu arms& pedals (two separate cly's) about $150..very nice.

[C R Box]

Thanks for the input, Norm.
Sorry but all brakelines are already braided hose, in movement areas.The front and rear ciruits have their own 5/8" dia clinders linked to the pedal via a (seems crude to me) balance bar.I think that this is effectively an older version of the 'Tilton' unit.
Incidentaly, I disassembled the pedal unit recently and noticed that the balance bar pivot was way off centre. Super mechanic (me) recentralised the pivot to see what the difference would be. Suffice to say that braking for the first bend of my favorite testing road, Healey and I virtually did a Thelma and Louise as we all but understeered off into the wild blue yonder. Fortunately I had the presence of mind to record the previous setting, which is where it remains!

[norman-normal]

C.R. if there is no air in your line, I think your pedal is about as firm as it's going to get. On my Supra W/factory disk F&R I could put the pedal on the floor untill I installed the SSbraded lines.
I was racing TR2 when the TR3(w/ft disk) came on the market. I up-graded the TR2 and was surprised at the pedal softness and travel. Triumph was the first with disk so there was no one to advise us about it.
The experiance is doubled with both FT&R. Your braking should be good.

[boyracer]

I forget exactly where I heard/saw it being used but there is such a thing as a residual line pressure valve. Used exclusively on automotive brakes to prevent 'knock-off', not sure of it's operation, knowing only that it holds a small line pressure (10 psi ?) in the line once the pressure is released to stop the seals or the wheel pushing the piston back.
On the rally car I service for we had a problem with brake knock-off, which became more noticeable with a tyre having lots of lateral grip. Since then we have installed springs in the callipers and the problem has disappeared. Note that the piston or the caliper body needs to have a recess machined in to allow space for the spring to collapse into when the piston is pushed all the way back (new pads). The spring is also a conical shape to aid this.

I do also remember a problem a fella had with brake knock-off and they cured it by going to a different seal style. This seal apparently didn't slide over the piston but moved with it (groove to wide ??) and once the pressure was released the seal friction on the piston would drag it back. I think this was the exception rather than the norm.

[Bomn]

A car that I worked on had residual line pressure valves. The driver said it gave a good firm brake pedal but the car was a mongrel to push around the paddock.
Try

http://www.wilwood.com/products/mast...rs/rpv/rpv.asp

or

http://www.pitstopusa.com/PROPORTIONINGVALVES.asp

[C R Box]

Thanks Boyracer and Bomn. I have previously contacted Wilwood about their residual pressure valves. The replied, saying it wouldn't solve the problem.I can't recall the techicalities. I'll try again but the point about not being able to push the car around is valid.Maybe I should give in and try the spring idea.The seal idea has already been mooted by a specialist garage. It could indeed be, that when the pedal is released the seal 'memory' returns the pistons to a position away from the disc. I've actually tried pre-loading them. i.e.having them advanced, so that when the pads are inserted, they force the pads back. It was fine, only for a few miles.
I still think it's knock off, due to the axle float.I think the pad springs are the best idea, although I just can't accept they actually work. Nex time it's in bits I'll fit them and renew the seals aswell.

[Morris 1100]

I would say that you have to reduce the axle float....

Is there a way of preloading the bearing with shims?

Or go back to drums!