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Cobra brake booster swap

Russell233

New Member
I have a 1987 LX 5.0 that I did the 5 lug conversion on w sn95 front discs. Still running rear drums. I recently swapped to the Cobra brake booster and now I have very low brake pressure. I didn’t swap the master cylinder. Was told I didn’t need to unless I went to rear discs as well. I have bled all the lines and the master cylinder. Still no real pressure. I’ve since plugged the line going into the booster and the brakes are actually better this way. Do I need to swap to a Cobra master cylinder or am I missing something else ? Thank you all in advance.
 

broncojunkie

Well-Known Member
Did the brakes work ok before doing the swap? You didn't mention it, so I presume they were fine. Otherwise, I would suggest checking for leaks, bench bleeding the mc, or replacing it.

So here's my suggestion presuming everything was fine before.

I have heard both ways. Some say the 87-93 mc works ok and others say the bigger calipers require more fluid movement. I would guess the truth is somewhere in between, and the difference you're experiencing is partially due to an out-of-adjustment rod. You can try adjusting it (or measuring it, although I didn't have much luck trying to do that with the amount of precision required.) My rod was adjusted too long with the cobra mc and booster (sn95 brakes). They worked perfectly for about the first 15 minutes of driving... then the front brakes would start dragging/sticking. Adjusting the rod is pretty easy, so you might try it. Remove front drivers side tire. Remove both nuts holding the proportioning valve in place (in the inner fender area). Remove both nuts holding mc to booster. Pull out on the proportioning valve till studs are clear, then pull mc out far enough to access the pushrod.

You will see the adjustment nut on the tip of the rod. You'll need a small pair pliers/vice grips to hold the rod and a small wrench to turn the nut.
 

Russell233

New Member
Thanks for the reply. The brakes were working before I swapped to the bigger Cobra booster. I believe the rod is adjusted correctly and I did bench bleed the new mc and each line. The brakes do apply but you need to go to the floor to stop and they won’t lock up enough for a quick skid. Im still running drums in the rear along w the sn95 in the front. Wondering if swapping to the Cobra mc will rectify my issue. I figure someone on here must have the same set up.
 

broncojunkie

Well-Known Member
I haven't ran that combination personally. It does make sense that the bigger pistons would require a bigger mc, but I'm surprised it is causing this big of a problem. That's why I suspected the push rod may have been a little short. The booster itself, I wouldn't think would make a difference as far as pedal height. It's my understanding that a bigger booster would just affect pedal effort... not cause the brakes to actually be "soft" or "go to the floor" in that sense.
 
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Russell233

New Member
Thanks again. I’ll probably go ahead and swap out the mc for a Cobra. Sucks because I just bought this mc because I suspected there was a problem with the older one . Now I have 2 that are probably perfectly fine.
 

broncojunkie

Well-Known Member
Did you run the factory booster with the new front brakes? You stated that you recently switched to cobra booster, but I wasn't sure if you did that the same time as you swapped calipers.
 

Russell233

New Member
Yes, I did run the factory booster with the new front brakes for a few years with no issue. My booster eventually needed to be changed as it was starting to go bad on me. I read many posts where people praised the bigger Cobra booster and figured I’d swap to one of those. Also would accommodate rear discs in the future.
 

broncojunkie

Well-Known Member
Like I said earlier, your booster, from my understanding, is only there to assist the hydraulic system. I larger booster will make pedal effort easy, while a smaller booster will make the pedal effort more firm (but require more pressure from your foot to engage the brakes). Neither should cause you to loose hydraulic pressure, which is what it sounds like you have going on.

This is why I suggested the pushrod length might be your issue. I only stated that, of course, because you said you properly bled the brakes and mc already.
 

nick bruscella

New Member
When I did cobra brake upgrade last year I had a low pedal also,I bled the cra p outta every caliper and still a low pedal.Then I rente- bled the master but no help.Someone suggested the master was bad,it was new?Well it had a AutoZone warranty so I got a free relacement,and what the he'll do youknow?My pedal was high and brakes were fan f ing tastic.. Just my two cents.let us know how you do.IMG_20200417_165439_hdr.jpg
 
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