Spongy Brakes

[Monzter]

Ok, I bought my Suburban the weekend after Thanksgiving 2007 & it needed rear brakes badly & the pedal would go almost to the floor before it would stop the truck, if I pushed hard enough I could put it into the floor easily. When I pulled the drums off both shoes were wasted ( no lining left at all ) and they wore into the drums, so badly it blew out the passengers side wheel cylinder. So a couple of weeks ago I swapped out to disc brakes in the rear, put rebuilt calipers & new pads on the front,  bleed the brakes but still the pedal is still spongy. So yesterday I bought a rebuilt master cylinder, bench bleed the master before installing it, & I rebleed the brakes on the truck after installing the master & I've still got a spongy pedal.  When I open the bleeders with someone holding the brake pedal down I get a strong stream of clean fluid & no air at each corner. So I've changed everything in the hydraulics but the metering valve, but if it was bad I don't see how I could be getting fluid under pressure coming from the bleeders when bleeding the brakes. Does anybody have any ideas on what could be wrong? This is not my first time doing brakes, or a rear disc brake conversion, but this is the first time I've had this much problems getting a firm brake pedal. 

[JJSZABO]

The propertioning valve is different for all wheel disk brakes than it is for front disks and rear drums.  You need to get a new propertioning valve that is suited for all wheel disk brakes.

See propertioning valves under this site:
http://www.inlinetube.com/

Are you sure you have no bubbles in the fluid?

[VileZambonie]

Yep, a combination valve consists of metering, proportioning and a differential pressure switch all in one and that's what these trucks came with. If you converted to a quick take up master cylinder and rear disc brakes but still used your combination valve you're never going to achieve the correct brake bias. The metering valve delays the engagment of the front brakes so that the rear shoes can overcome the spring seat pressure of the shoes. So a disc disc system should not have a metering valve. Depending on the master you put in you need a proportioning valve for correct brake bias however neither of these should affect the pedal to where it goes to the floor. If you bench bled and bled the system corretly and are positive there are no external leaks either your brake pedal pushrod length is wrong for some reason or you have a defective master.

[Monzter]

I under stand how the metering vale works, I was going to see how the brakes worked with the discs in the back before I made any changes to the system. I was kinda thinking I got a bad Master cylinder, but it's odd how it would feel just the same as the old one. I'm gonna return the master cylinder tomorrow & put another one in to see how to works, while it's out I'll check to see if the pin between the booster & master cylinder is the right length. If that don't fix it I'm gonna remove the metering valve to see if that will firm up the brakes. I was thinking about removing the metering valve anyway & using this one from Summit. http://store.summitracing.com/partdetail.asp?autofilter=1&part=SUM%2DG3910&N=700+4294918718+400304+115&autoview=sku

[VileZambonie]

You shouldn't have a metering valve if you have disc/disc so you definitely need to bypass your stock combination valve. What application M/C did you use? You'll need an appropriate proportioning valve for sure.

[Monzter]

Got it fixed.    The rebuilt master cylinder was no good. The brakes work way better than stock, even with the stock metering valve. Pedal is good n' firm & it will lock up all 4 brakes when push hard enough, which ain't much. Today was a good day to do some panic stop testing too with the roads wet, don't think I need to change anything. The master is for the original drums in the rear, I haven't had any problems using this master cylinder on disc swaps in the past. Usually I'd have to install an adjustable proportioning vale though when swapping to rear discs, but not in this case, there's a lot of weight in the rear of these Suburbans.