Master cylinder/brake booster trouble

[MoabXJeeper]

So this weekend I thought I would try my hand at replacing the stock booster with a dual diaphragm booster on my 92 XJ. I got the stock booster off and put the new booster in place, but quickly realized the new booster wouldn't work with the old style master cylinder (MC) as the increased size in the booster pushed it right up against the air box. I didn't have the newer style MC on hand and need my Jeep for some trails next weekend, so tried to put the old booster and MC back on. Which is where the problem began..

I had separated the MC off the old booster when I took it all off to see if it would work with the new one. Those bolts were on there TIGHT. Well, I got the old booster back in place, put the 4 bolts back on and put the pin back into the rod on the pedal side, bolted the MC as tight as I could back up to the booster, put the brake lines back on, and put the vacuum line back in. Everything looked exactly as before I had taken it off.

The problem is, the pedal goes straight to the floor now without hardly any resistance.

Things I tried:

1 - Bled the brakes, no air came out, just a steady stream of fluid still came out as normal. Refilled the MC with brake fluid each time.

2 - I turned it on to see if it needed to build up vacuum pressure, that didn't do it.

3 - I had my dad pump up the brakes while the engine was on and looked if I could see anything. I noticed at the very bottom of the MC, there were some drips of brake fluid coming down on the front side of the booster, like it hadn't sealed shut. I backed the two bolts off that hold the MC and re-tightened them, trying to make sure they were completely level when I tightened them. Same thing happened.

Anyone got any ideas? I'm pretty sure it's just the MC not being completely sealed up to the booster, thus not allowing for any vacuum pressure to build up. I'm just not sure what to do about that? There wasn't anything like a gasket between the two when I pulled it off the booster the first time. Really wishing I would have just left the MC on there now..

Any suggestions would be greatly appreciated.

FYI, my brakes worked before, they just didn't stop 35's very well, so that's why I was trying to do the dual diaphragm booster.

 

[Ecomike]

There is no seal between the MC and Vacuum booster, never has been, it is not a seal area. Good news (maybe), your vacuum booster (old) may still be OK, but if the old MC leaks fluid out the rear, it is toast, and the fluid will ruin the Booster if gets inside the booster. If has been leaking all along, the booster will soon leak if it is not already leaking vacuum.

Did you bench bleed the old MC before reinstalling it? That is 200% must always do thing!!! Then it seems you need to bleed combination valve which is between the brakes and the MC. Search here for a 2012 thread of mine using "Ecomike 87 brake bleeding" about the combination valve bleeding issues. They seem to trap air, as does the MC, if not bleed properly first.

The vacuum is NOT needed to bleed the brakes.

Have you ever bleed brakes before?

 

[old_man]

I went with a dual diaphram booster instead of a single diaphram, and it definately needed to be sealed to the MC.

There is no seal between the MC and Vacuum booster, never has been, it is not a seal area. Good news (maybe), your vacuum booster (old) may still be OK, but if the old MC leaks fluid out the rear, it is toast, and the fluid will ruin the Booster if gets inside the booster. If has been leaking all along, the booster will soon leak if it is not already leaking vacuum.

Did you bench bleed the old MC before reinstalling it? That is 200% must always do thing!!! Then it seems you need to bleed combination valve which is between the brakes and the MC. Search here for a 2012 thread of mine using "Ecomike 87 brake bleeding" about the combination valve bleeding issues. They seem to trap air, as does the MC, if not bleed properly first.

The vacuum is NOT needed to bleed the brakes.

Have you ever bleed brakes before?

 

[Ecomike]

I went with a dual diaphram booster instead of a single diaphram, and it definately needed to be sealed to the MC.

Interesting. I wonder why they changed that? The 85, 87 and 89 I have are all rough cast surfaces with no gaskets, no signs of gaskets, and no signs they were ever meant to seal.

In fact two of my boosters failed when the Vacuum booster sucked brake fluid out of the back side of the MC into the BB guts!!!!:tears:

One time was a rebuilt MC (lesson learned, only buy new). Next time was a bad BB that had already been damaged by the bad MC leaking out the rear so the BB died, and I just replaced it all that time, third time was a short lived aluminum POS MC (Not anodized, it corroded badly inside and outside in months). I only use cast iron MCs now.

I'd like hear and see more on this seal-no seal issue of that area between brake MC/BB generations!!!! With all the issues I have had, I would think they would want an open air vent between the two!!!! But what do I know? LOL!!!!

 

[old_man]

It may be too long ago but I did a writeup and conversation about the dual booster upgrade. It was probably 10 years ago. I used an E350 MC.

Interesting. I wonder why they changed that? The 85, 87 and 89 I have are all rough cast surfaces with no gaskets, no signs of gaskets, and no signs they were ever meant to seal.

In fact two of my boosters failed when the Vacuum booster sucked brake fluid out of the back side of the MC into the BB guts!!!!:tears:

One time was a rebuilt MC (lesson learned, only buy new). Next time was a bad BB that had already been damaged by the bad MC leaking out the rear so the BB died, and I just replaced it all that time, third time was a short lived aluminum POS MC (Not anodized, it corroded badly inside and outside in months). I only use cast iron MCs now.

I'd like hear and see more on this seal-no seal issue of that area between brake MC/BB generations!!!! With all the issues I have had, I would think they would want an open air vent between the two!!!! But what do I know? LOL!!!!

 

[MoabXJeeper]

Have you ever bleed brakes before?

I haven't ever bled the master cylinder like that, no. I just did a normal brake bleed was all.

 

[Ecomike]

I haven't ever bled the master cylinder like that, no. I just did a normal brake bleed was all.

Bench bleeding the MC first is always a must. Otherwise they never get fully bleed (or so I am told), as the mounted position does not get a full stroke.

I air gets into the combo valve it will never make it to the brakes, it hides in the valve. I had to uuse the front side fitting (accesses the valve guts) and loosen it then have a helper push the peddle, and hold it while I closed the front fitting, then repeat....Then bleed the 4 wheels last.

 

[old_man]

Personally I use an electric vacuum bleeder and have never had a system I could'nt bleed.

 

[Ecomike]

Personally I use an electric vacuum bleeder and have never had a system I could'nt bleed.

LOL, you never tried to bleed my POS!!!!!

That said, I never had trouble bleeding anything till I got a JEEP!!!!:banghead: