Rears locking under heavyer braking

[mikie]

Hey I'v got a RA40 that is constantly locking rear brakes (no I’m not using the hand brake). Iv noticed this after I swapped wheels from the stocky 185/75/14 to 225/65/14. the rubber on the wheels seems to be old and hard and my guess would be that the problem lies there. I have not done any brake modifications lately, which includes new pads or shoes , I have bled the system already with no change.

What do people think???

Cheers Mike

[leso]

you may be having a problem with your proportioning valve...unsure of where it is located on a ra40...had a similar prob on my mates hq recently....they normally sit just below the brake master cyl...and distrube the fluid to the front and rear brakes...

cheers leso

[rollin_stock]

Have you maybe just not noticed the problem till now because of the tyres, or did you only put different wheels on the rear? I dont think a whole set of tyres would upset bias. Have you reaplaced rear wheel cylinders lately at all, could be a problem with them being too small or for the wrong application. Otherwise as leso has said, check the proportiioning vavle.

Could also be a rear shoe sticking on.

[mikie]

ill check the valve next chance i get, as to the system completely stock nothing has been changed from master to calipers or rear cylinders. The tyres are 195/65/14 front and 225/65/14. the rears are at least 4 years old, and haven't been driven on till recently.

cheers mike

[rollin_stock]

So you did only changed the rear wheels?? i guees this is your problem, fronts should always lock first on a unmodified vehicle. Put the original wheels on the rear and then do the same hard brake, fronts should lock, otherwise yes you have more of a problem.

[Karma Supra]

I have the same problem on my '83 944.....

sure wakes you up first time you need to make a sharp stop!

[CrUZida]

There should be enough fat in a factory system such that if you do put crap tyres on the rear, the fronts will still lock up first.

While you may have only realised it now that you've changed tyres, it doesn't mean the tyres are the cause of it. It could just simply mean that the problem has always been there, and the tyres are just making it more obvious.

It might be worth getting a brake bias test of sorts done.

[thechuckster]

it could also be that the crapulous RA40 front brakes are not working that well (e.g. frozen piston, crap pads, etc) and the lack of traction the new rears is merely exaggerating the problem.

[rob]

yeah my RA40 locked the backs as well, it didnt bother enough to look into but it always felt that the rear end was way too light, also never gave me much confidence on long fast sweeping corners because of it, particularly one bend that i could take just as fast in the stock KE35 because of its corrigated surface.

[EVOSTi]

why not put in an aftermarket prop valve and be done with it, wilwood ones are under $200 from memory and will be usefull if you ever decide to change brakes in the future.

[ViPeR_NiPPleX]

an adjustable valve would need to be engineered wouldn't it?

[rob]

i thought they were illegal on the road

[EVOSTi]

ive never spoke to an engineer about it, but the consensus is they are legal if they cannot be adjusted from inside the cabin.

[takai]

They are only illegal if they are in-cabin and adjustable while driving. If they are in the engine bay they are fine. Some engineers like to see them with a little plate covering the knob so you cant bump it accidentally.

[CrUZida]

It shouldn't need to be engineered either, just certified by a brake shop.