new master cylinder brakes are delayed and hard to push.. vacuum??

[HSkinner]

I recently replaced the master cylinder with an autozone unit only because I was in a bad way and they had it. They asked me if the truck was over or under 8500gvw and I said it was over and I now realize that an 86 ext cab f250 is not over 8500gvw. I bench bled it and then still bled it at all of the wheels. I've been driving it on and off for about a month and there is definitely something off. When I press the pedal it is a delayed reaction and the pedal is very hard to push. I checked for obvious vacuum leaks and couldn't see anything. Is there anything I can do to fix this? Can I connect the vacuum pump directly to the booster to determine if there is a leak somewhere in between? I've read a little about the hydroboost conversions but I would prefer to figure this out first before going that route. Is the master cylinder the cause of this? There is a coffee can looking canister mounted on the inner fender well on the driver side. is this factory?

 

[HSkinner]

Also, my auto trans requires the rpm's to be up there a bit to shift. Does it sound like I need a vacuum pump?
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[79jasper]

Get yourself a vacuum tester. Say like a mityvac.

Hard to push usually means the booster.
Boosters do go bad.
A good rule of thumb, If the old mc was leaking, consider the booster done for.

Compare pedal feel with engine on vs engine off. If equal, it's in the booster somewhere. Rather it be the booster itself, or a vacuum problem.

Sent from my USCC-C6721 using Tapatalk

 

[wildman7798]

When I changed my M/C I didn't know which one I needed so I bought them both. There may be a bore size difference but they had different sized flared nuts so I don't think you could have put the wrong one on. The can is a vacuum reservoir or storage can. When I got my truck I had a similar issue, I noticed the pump belt was flopping around and in turn found the vacuum pump bracket was broken in half preventing enough belt tension to give me full vacuum. As stated you need to get a gauge on it so you can see what is going on. I don't have the exact number these pumps put out but it seems like I recall 17"-19" which is pretty similar to a gasser. A few things you need to rule out before you start looking at a booster.

 

[yARIC008]

Either you're losing vacuum which is possible if you knocked a vacuum line loose while changing out the MC or you got the wrong master cylinder. It is very unlikely that your vacuum pump just happened to fail at the same time. It's more likely a line got knocked loose.

You didn't really say why you decided to change it in the first place, so maybe your vacuum pump was already going out and you changed out the master cylinder because of that?

If you got a master cylinder with a larger bore then your booster will not be strong enough to give your the boost you need and your brakes will feel hard.

So basically, either the vacuum not there for whatever reason as stated above or wrong size master cylinder...

 

[OLDBULL8]

Vacuum is easy to check at the booster, with engine running, pull the 90* fitting out of the booster, you'll hear it sucking in air if there is vacuum on it, you can also just disconnect the hose, if it sucks in air, then that 90 is bad, the 90 is a check valve. To check the vacuum with a gauge, the vacuum should be at least 24hg. no lower than 20hg to make the tranny to shift right. The tranny shifting is controlled by the VRV on the IP, it may be set wrong, or the modulator on the tranny is defective.