Brake Bias

[hsv-001]

Can someone explain why it is so difficult to get anyone knowledgeable when it comes to brake bias .Just had a TAC inspector check my hotrod chassis and believes that there maybe something wrong with running a VR commodore booster on a drum rear disc front setup but couldn't tell me why . After reading many forums [both torana and commodore] and many pages of specs.[hoppers etc] ,I can only determine that there are three proportioning valves in the later commodores .The lighter being for the ute at 225 and the heaviest at 450 [I"m guessing V8] and 350 in the V6 and in the latest of the utes load sensitive self adjusting [ a real problem if you lower the vehicle].Back in the day we used varying wheel cylinder sizes to make sure that the rear brakes didn't come on more than the front .Too large in the rear and they locked up before the front and tended to put the vehicle sideways and the fluid displacement could make the peddle spongy or travel too far. Too small on the rear and the back brakes are not effective . Does anyone out there know the proportioning valve sizes in comparison to the wheel cylinder sizes in the drum disc toranas and HQ to early drum disc Commodores ? I would also guess that brake bias would a lot depend on drum shoe and rotor pad quality . Hope someone can enlighten me .Cheers Haydn   

[FCRB26]

Cannot give you too much insight into sizes ect but i recently did the brakes on a mates torana drum rear end hq fronts i used a 1 inch wilwood master cylinder front and rear and ordered a certan percentage wilwood bias valve cannot remeber what it was.
Summit said this is the one to use for drum rear ends using a 1 inch master.

Dont commodore utes have a load sensor on the rear like most 4x4s that changes the valve to suit the weight of the load you are carrying ?

[hsv-001]

As I said this must be considered when lowering your ute . But no , I am simply trying to use a VR sedan master cylinder on my Vauxhall hardtop , exactly the same torana front and rear ends as the convertible and it works fine on the convertible for the last 5 years [put the inspector up against the windshield back then] but he said to me I should ask someone who does brake if it is ok . So my brake guy [if not me] has retired after some 25 years [he used to ask me about these mix and match jobs] and now I can't find anyone who makes sense . I would be inclined to think that a disc disc  valve off a sedan would favour the front slightly more than a disc drum valve and would therefore work well as in my Vauxhall there would be on balance more weight over the front than a commodore . But as usual I may be wrong .Haydn

[fe1957]

As I said this must be considered when lowering your ute . But no , I am simply trying to use a VR sedan master cylinder on my Vauxhall hardtop , exactly the same torana front and rear ends as the convertible and it works fine on the convertible for the last 5 years [put the inspector up against the windshield back then] but he said to me I should ask someone who does brake if it is ok . So my brake guy [if not me] has retired after some 25 years [he used to ask me about these mix and match jobs] and now I can't find anyone who makes sense . I would be inclined to think that a disc disc  valve off a sedan would favour the front slightly more than a disc drum valve and would therefore work well as in my Vauxhall there would be on balance more weight over the front

Hi Haydn, the only thing I would say you might need is to adda residual valve, willwood make them $25 I think

Residual valves are pressure valves used to retain pressure in the lines. The most common use is on a hot rod when there is a floor mounted brake pedal and master cylinder. Mounting the master cylinder (M/C) below the floor positions it below the calipers. Gravity will cause the fluid to flow away from the calipers. The residual valve will retain pressure within the lines. (i.e. 2 pounds residual valve will retain 2 pounds of pressure, 10 pound will retain 10 pounds.) Drum brake master cylinders have residual valve(s) built into the master cylinder. This is needed to maintain pressure against the cup seals in the wheel cylinders. If you are using a disc brake master cylinder or after market you will need to install a 10 pound residual valve for the drum brakes. Do not install a residual valve if your master cylinder already has one in it. This will cause the brakes to lock up after the second application to the brake pedal.

Cheers g

[fcwrangler]

Haydn, Ken (mcl1959) would be the one to chat with, he seems to be the go to guy when it comes to brakes.
Regards Jim

[hsv-001]

Yeah, ok so this residual pressure valve is the little bowler hat shaped valve you take out when you use a drum brake master for a clutch or disc brakes with say a HR front end . So will I need it to put a VR booster and master [fire wall mounted] on a disc drum combo ? Like I said ,it works fine on my first Vauxhall . Haydn

[fe1957]

Yeah, ok so this residual pressure valve is the little bowler hat shaped valve you take out when you use a drum brake master for a clutch or disc brakes with say a HR front end . So will I need it to put a VR booster and master [fire wall mounted] on a disc drum combo ? Like I said ,it works fine on my first Vauxhall . Haydn

It gives you a better pedal feel, it you have rear drums adjusted well as you say will work aok.
G

[hsv-001]

Ok I think I got it , If the brakes feel spongy its because the wheel cylinders are being pulled[purged of fluid] back to virtually empty by the return springs [if not adjusted correctly]. If I need one of these valves [still not convinced I need one ] I should start by trying a smaller one as too much pressure retained will result in rear brake lock up. Thank you everyone for your help . Now I can get back to my build without worrying about that curve ball. Til the next over zealous inspector demonstrates his extensive knowledge . Cheers Haydn