rear brakes

[sonscar]

My car has Princess 4pot front brakes with vented 255mm discs.Hardly believable but I have not driven it in the rain hardly,until Saturday.The brakes have always seemed sensitive and the fronts lock up easily under modest pressure,but in the rain you hardly dare use them.I know that I should have bought premium quality tyres but my wifes modern daily has the same cheap tyres and they grip tenaciously in the wet.
Reading extensively on the subject leads to more confusion,the rear cylinders are 5 speed ones and research suggests that the larger(or smaller depending who has written the article)4speed ones be fitted.
At a loose end I found some MGB cylinders in my stash,a quick measure shows them to be larger than the existing ones so some new piped and they were fitted.
Time for testing being short I only did a few miles but this showed the fronts still lock and the rears do not,which is good?but only a few miles will tell if the issue is adressed.
Am I on the right track?
P.S. To cap it all the oil light is now on almost permanently.I have fitted a guage and the pressure is 30psi at idle so I guess it is the sender.I do not know what it is when driving as it is under the bonnet.
Sorry for the essay.Steve..

[Graham.Fountain]

I've had these princess 4 pots on several versions of A TR7 16V, and they're better then original ones, but they weren't especially prone to locking. So if the fronts are locking very easily on a dry road, I'd look at whether there's a problem with them grabbing.

Not sure if it's relevant, but with standard disks you have to move the mounting holes on the calipers, to stop the pads hanging over the edge of the disks? I know it should make no difference to their stopping power, because that's not a function of contact area, just force and pad to disk CoF. But if the edges of the disks are rough, and the pads are hanging over, that could cause a bit of grabbing while they wear smooth. But with this size disk I wouldn't have though that a problem.

What it says in the old Triumphtune manual is to fit the smaller 4 speed rear cylinders. This is because, as you increase total brake force, the ideal brake ballence point moves forward. However, that also involves an increased coefficient of friction, because you can now lock the fronts at a higher CoF value. So whether it's actually necessary without you've also gone to stickier tires is an interesting, i.e. difficult, question. But what you lose at the back should be less than what you gain at the front, so it should be the safe option. But increasing the effort at the back could be a mistake.

Graham


TR7 Sprint VVC 697S (some of)
TR7 DHC Sprint A TR7 16V (fake, rusty):
B&Y '73 Doly Sprint (kids!)

[sonscar]

I refurbished the calipers with new seals and stainless pistons and was careful to ensure all 4 of the pistons moved freely,the discs are new and the pads do not overhang the disc at all.Braking is indeed a dark art and the information is contradictory as,I suppose,most is subjective.There does appear to be confusion over whether the 4 speed cylinders are larger or smaller than the 5 speed.Having some spare cylinders from my MGB roadster when I converted to discs I could measure them and they are definately larger.I will persevere a while and get a better feel for the new system over a more varied route than just up my street.Thanks for your insight,Steve..

[sonscar]

So after more miles on the altered brake setup the conclusion is;inconclusive.Initially there was more pedal travel as I expected due to the greater amount of fluid to be moved,but after some mileage and readjustment of the rear brakes(the auto adjusters do work but I always find that removing the drums and giving them a few more clicks tightens them up nicely although you do have to be careful as you have to manually back them off if overdone.)the travel is very acceptable.
The rears show no tendency to lock even in the wet,but it is still easy to lock the fronts as the system seems very sensitive,I guess I have lead feet.I always know that it is only the tyres grip that ultimately dictates lots of the driving dynamics,but at the end of a build I am always skint and end up buying economy tyres(false economy tyres?).Still drive within your limits and keep safe eh?.Thanks for looking,Steve..

[Hasbeen]

What did you do with the discs? Are they new? Are they refurbished, or old & well used?

Do you have a dial gauge available? If so set it up against each disc, out near the outer edge, & check them for run out. If you have much more than 6 thou, you need to correct it. I would not run with more than 4 personally, but many do, without problems.

They shouldn't be that touchy. I have a Volvo master, & Volvo 4 pot callipers. I have to use the brake very firmly to lock a front, & had to stand on the things to lock a rear, when it still had drums.

It might be worthwhile checking the distribution valve, it may not be passing enough pressure to the rear.

A bigger diameter cylinder will apply more pressure to the rear shoes. It is simply a matter of the pressure [per square inch] applied to a larger area of piston applies more to the shoes.

Hasbeen

[76tr7rules]

Hasbeen did you use the Volvo master with or with out a power brake booster? How hard was the install?

[Hasbeen]

With booster.

It was a kit supplied by a local supplier.

Hasbeen

[TR7TIM]

If you can't fix the problem, you can always fit an inline manual biased valve. These can be purchased from many places, including race and rally outlets. This valve allows you to adjust the amount of pressure to front and rear brakes. Also, what pipes have you got fitted? Rubber or nylon? I've had trouble with front in balance before due to old rubber pipes collapsing internally due to age. Maybe your rear one needs replacing?? Tim.

[sonscar]

All the system has been repiped and the flexys replaced with Goodrich braided hoses.The calipers are rebuilt and the rear cylinders new.An unscientific test reveals that (by hand)a lot of pressure is needed to lock up the rears but the MOT rollers showed the braking force to be acceptable(I know that this is not the same as being really as efficient as it could be).Some new quality tyres may provide an answer but I cannot justify the cost.I guess I need more skills.Steve..

[Hasbeen]

That's sure to be it Steve, clumsy right foot. [}:)]

Actually I would have a look at the distribution or proportioning valve, what ever it is called in your area. It is what decides how much pressure to give to each end of the cars brakes, not your foot. It may be giving the rears too little compared to the front.

There are after market adjustable valves, which could fix your problem, with a fair bit of mucking about, hooning up & down a secluded road somewhere.

Our V8 super cars have, or did have, brake force distribution valves adjustable from the drivers seat, to adjust the pressure front to back as the fuel load diminished during a driver stint.

Hasbeen