Spongy/No pedal, air still in system? Bad MC?
 

After replacing front driver/passenger brake hoses and lines (72 Blazer), and after multiple bleeds, I still have spongy pedal that eventually goes to floor. It won't build up after pumping, just keeps going down. I'm using clear tubing on the bleeders, and first or second bleedings I saw air bubbles, have not seen any air for last few bleeds, but still no firm pedal. I'm also using one of the screw-in tools on the proportioning valve. Next step would be to try vacuum bleed. Any other suggestions or things to try? Also wondering best way to tell if MC seals are still okay. I'm not seeing any fluid around the rod/plunger. And double checked fittings on lines/hoses, they are tight without leaks. Thanks for any help.., I've called it quits for the night.

Re: Spongy/No pedal, air still in system? Bad MC?
 

Removing and/or replacing any brake lines have allowed a lot air into the system. If you aren't replacing the master cylinder, the best way to bleed is to slightly open all the bleeder valves at the wheel cylinders and calipers.. Remove the lid from the MC. This will allow the system to gravity bleed.. Make absolutely sure you keep adding fluid to the MC and do not allow it to run dry.. After you have fluid flowing from all the bleeders, close them, fill the MC and replace the lid..

Now, forget hoses, vacuum bleeding and any other "trickery" you may have heard.. Have a helper sit behind the wheel. get in a position where you can see the right rear bleeder.. Open it slightly and have the helper shove the brake pedal to the floor and hold it there, while you close the bleeder. DO NOT PUMP THE PEDAL.. Release the pedal and repeat these steps 2 more times.. Fill the MC again and move to the left rear.. Do the same thing with it. Do the right front the same way, and then do the left front.. Always keep the MC full of fluid while you go through this process.. You should now have a good solid brake pedal..

Re: Spongy/No pedal, air still in system? Bad MC?
 

Okay, still struggling with this. No problem with gravity feed on the front, open the bleeders and fluid freely runs out. Not the same with the rear, barely a few drops. With top off the MC, I can see active flow of fluid from bottom of front reservoir shooting up when pedal is applied. I don't see the same in the rear reservoir. Proportioning valve is locked with screw in bleeding tool in place of brake light hookup. To check that fluid is making it past prop valve going to rear, I unscrewed line at back of prop valve and I can see fluid coming out when brake pedal is applied, but it's not a lot. So, should I be seeing that steady flow coming up from bottom of rear MC like I do from the front? Thanks again.

Re: Spongy/No pedal, air still in system? Bad MC?
 

Check your PMs..

Re: Spongy/No pedal, air still in system? Bad MC?
 

It's been a while since I did the brakes on my 70. I swear by these vacuum bleeders.
https://www.harborfreight.com/brake-...kit-63391.html

If i recall, I had trouble vacuum bleeding the 70 specifically. i had to prop a board on the brake pedal and depress the brakes a little bit for fluid to start flowing.

Re: Spongy/No pedal, air still in system? Bad MC?
 

Sorry box was full. It should be clear now. Thanks.

Re: Spongy/No pedal, air still in system? Bad MC?
 

Ok, I sent you another message

Re: Spongy/No pedal, air still in system? Bad MC?
 

I changed to pressure bleeding a long time ago.
I suppose if you only have to bleed one vehicle every so often it may not be worth it, but it's so much easier to do, especially by yourself.
Attachment 2272744

Re: Spongy/No pedal, air still in system? Bad MC?
 

Make Shure your rear brakes are adjusted up so you have minimum clearance between the shoes & drums. If that is OK you probably have a bad master cyl. They often have internal leaks that don't leak to the outside. Its a good idea to replace the master cyl. when doing brake work if it hasnot been replaced for a long time. The crud & corrosion have built up in it & when you start moving the pistons down into the unused portion they will start leaking or can draw air in on the up stroke.

George

Re: Spongy/No pedal, air still in system? Bad MC?
 

I've been away and just got back to this today. It was the master cylinder. I replaced it and bled all the way around with any weirdness. Great pedal and stopping now.., night and day from before. Thanks again for the input/help.