rear discs brakes

[rizzo]

I have a friend who put rear dics on his truck but is not getting enough pressure to the back discs. when driving he can lock up the front but not the rear.

all calipers are the same and he has a 1ton hydroboost
he used a 1/2 ton proportoning valve and went straight to the master cylinder. Neither worked.
If he jacks up the rear tires the brakes will stop them, but they won't lock up when driving like the front ones do.

any ideas?

 

[dilvoy]

Disk brakes usually move more fluid than a drum brake wheel cylinder. That being said, I have converted my 98 Dodge Ram to rear disk brakes and it is using a caliper from an '90's caddy and it works well. The rear abs actuates from time to time, so I know it's working fairly well, but I was concerned with the possible lack of stopping so I positioned the loaded/unloaded proportioning valve in the loaded position and the breaks worked much better. They were more responsive, but I went through those pads on 10,000 miles so I put the linkage back on the load proportioning valve back on and the brakes stop just like when I had the drums installed, but I don't have to adjust them all the time.

 

[rizzo]

I understand. It sounds like the best thing to do is find a master cylinder that will hook up to his hydro assist that is meant to be used for rear discs?

right?

any ideas?

 

[gasser]

I not sure how relevant is this, but ....
I put rear-discs on my SS chevelle (non-power), and I used a non-power 4-disc master cynlinder from a mid-70's Corvette and portioning valve from Wilwood. Front and rear disc kit from was Wilwood.

 

[TheBuggyman]

Dilvoy is absolutely right, a disc brake caliper requires more fluid than a drum wheel cylinder so you must put on a master cylinder from a four wheel disc application to get enough volume to the rear. I have converted several systems this way and sometimes you need the proportioning valve and sometimes you don't. (most of the time you do!)

 

[AJMBLAZER]

Might want to check with the kit's manufacturor unless it was a home build. Hell, call up Off Road Design and ask them what they recommend.

 

[Taz]

I put rear discs on my 1008. There is a check valve in the porportioning valve that restricts fluid flow to the rear brakes. It unthreads out after you remove the rear brake line from ithe valve. I removed the check valve inards and it works fine. I have to say it would be better to install an adjustable proportioning valve in the rear line and tee off to the fronts as there is a little too much rear braking on icy roads. Good luck

 

[rizzo]

my friend said he bypassed the valve and went straight to the master cyl. still would not lock up on snowy road.

 

[CCATLETT1984]

if he went straight to the master cylinder and they wont hook up, there is something in the line or still air.
I would re-bleed the system again. he is still using dot 5 fluid, correct? If he mixed in dot 3, I'll bet he has some nice looking goo in the system (ask me how I found that one out, darn guy I bought the truck from)

 

[jimk]

My thought too.Make sure they are bleed.You can test each caliper's pressure by making screw in tester .Join a cheap 3000psi gage and a old bleeder screw.750psi-1500 normal.

Brakes disks require more force that drums (drums self energize).The area of the piston(s) determines force(and ,of course, the master cyl dia). Caliper pistons are huge(dia.) compared to wheel cylinders. A 1" wheel cyl (drum) is just over 3sq inch (1"x3.14(aka pi)=3.14sq inch). A 4 piston caliper with 2.5" pistons has 31.4 sq inch and generates 10 times the force(2.5"x3.14x 4=31.4 sq inch).At, say, 1000psi the drum gets 3140lbs of force while the caliper makes 31,400lbs.(just made this up)

The GTO uses metallic pads(front).They need to be warm to work well. A few seconds is enough for the rest of the drive. Seems the hotter they get the better they work.I used a std disk/drum master with the 4wheel disks.I made a new hole in the foot pedal arm to increase the force at the expense of travel(more).Pontiac did use a 4wdisk master with a 3rd large piston,called quick take up,to move te pads fast until contact with the disk.I never needed it.Various master dia. may be a useful tuning aid.

I have a Kelsey Hayes adjustable proportioning valve on the 4/w disk GTO.Similar to the Willwood. It is mounted under the drivers seat and reduces pressure to the rear brakes to prevent early lock up.It varies the amount, automatically, as pressure goes up (past a certain pressure point)

If you have a in line check valve make sure it goes the right way. It traps a bit of pressure(2-5psi) to keep the pads near the disk (so pedal travel is kept to a min).Sometimes used to keep the fluid in place in a system where the caliper is higher that the master(racing).I do not use one.

If every thing is in order you may need to use a proportioning valve for the fronts.An adjustable one give the most control and may work best with a system that did not get designed from the beginning .

JimK

 

[TOBEXA]

what size are the rear drums on the 1009? 13?

 

[TOBEXA]

if you had to replace the rotors, calipers, and drums. would you go with a different kit or just keep it stock?