Is it only the rears that lock up or all 4 corners?
I would check your brake line psi at the backing plates.

 

cant tell but it feels like all 4, id have to watch someone do it to know

 

i know this sounds crazy but it almost sounds like your getting too much pressure, i had a problem on my 68 that if i hit the brakes in the rain it would yank me to the left front and the left front would lock sometimes, i did some readjusting and it fixed the problem. basically i had more pressure going to the left front than anywhere else, i figure had i had all four set like that i woulda locked all four up in the rain.

 

That's one of the drawbacks with a four drum power brake set up. When you apply the brakes the weight distribution of the vehicle transfers to the front, thus lightening the rear. In low traction situations like wet roads, your rears will lock up extremely easy. You need a proportioning valve to dictate more pressure to the front brakes and less to the rear. Better yet would be a proportioning valve with disc brakes up front. If you actually have a proportioning valve with the Geo set up, an adjustment to lighten the rear pressure will help, but may also cause the fronts to lock up easier.

 

I have been driving my car in the rain for the past few days. No locking up except for in a parking lot, when some idiot came speeding out of an isle.

I drive on roads varying from 45-25. No locking yet, even with the surprise red lights. I just have to be sure not to press them too hard too quick. If I work my way up in pressure, they don't lock. But stomping does it.

Anyways, I have 4 wheel manual drums, the entire system rebuilt 1 year ago (returned drums, new wheel cylinders, new master cylinder, and of course shoes and whatnot)

 

While drum brakes are more prone to lockup in wet weather, there are some items that do promote it. To start, inexpensive brake linings (such as Raybestos, Ford replacement units, etc.) are not spec'd to the same requirements as the real original equipment causing reduced braking (about 20%) and lockup when wet. Replacement with a higher quality lining will help considerably (such as Wellman, Carbotech, Hawk). Additionally, if the lining is one continuous strip of material, the installer should take a hacksaw, cut blade width gooves diagnally ( 2 to 3 on each shoe) to channel water to the backing plate. The gooves should be no deeper than 1/2 the thickness of the lining with the edges lightly sanded so not to create any vibration.

 

Part of your problem is the MC. The GeoMetro had front disc's and the MC is setup to push more volume required
to move the pistons (vs. your much smaller wheel cylinders.).
Bob.

 

so from what ive gathered is that disc brakes and a proportioning valve would make all the difference in the world...correct?

 

Dino hit the nail on the head. I bet you are pushing too much fluid w that MC. IMO a disc-brake MC should not be used with a 4-drum setup.

 

A sliding (locked up) wheel tends to lead. So, if the rears are locking up, they will try to pass the front end. Sounds like this is what's happening to you. A proportioning valve for the rears may definitely be in order. A lot of power brakes are a little "touchy", and you will learn to deal with that, but the premature locking of the rear wheels is definitely a problem. Hope you get it "tuned in"!

 

yea i know about the disc brake MC pushing more fluid, i chose that setup because i plan on doing wilwood's up front soon. I also think the brakes need to all be readjusted i think that has something to do with it.

thanx for the info