Has anyone used a 7 or 8 inch dual diaphragm aftermarket brake booster?

[My427stang]

I need to replace the booster on my F100, its a, 8.75 inch diameter single diaphragm.  I was considering using one of the aftermarket dual diaphragm 7 or 8 inch models to gain a little valve cover room and get a little more stopping power.

Has anyone tried them, any downside?  Will a dual 7 or 8 push harder than a rebuilt original you think?  I assume you need to use a GM master cyl, but I can't see any other downside.

[turbohunter]

I've had a dual 7 inch and a single 8 inch on my 292 truck. I have not had any problems with either one. I have them on all drum brakes. I notice with the single 8 that there is a normal feeling application for normal stops but I've had a couple of panic stops lately (I live in LA, go figure) where I applied a normal amount of force and then had to press harder to really stop quick. The real hard press kinda felt like I was out of boost. The truck stops really well and there was no problem but we all know brakes pretty well and I figure you'll know what I mean. It may be because the 8 inch is a single and kinda ran out of gas. Again it wasn't a problem just something I noticed.

[My427stang]

I've had a dual 7 inch and a single 8 inch on my 292 truck. I have not had any problems with either one. I have them on all drum brakes. I notice with the single 8 that there is a normal feeling application for normal stops but I've had a couple of panic stops lately (I live in LA, go figure) where I applied a normal amount of force and then had to press harder to really stop quick. The real hard press kinda felt like I was out of boost. The truck stops really well and there was no problem but we all know brakes pretty well and I figure you'll know what I mean. It may be because the 8 inch is a single and kinda ran out of gas. Again it wasn't a problem just something I noticed.

Thanks, I haven't decided which way to go yet, Advance Auto replacement is by far the cheapest, Booster Dewey is the most expensive but a very good name, or go with the dual 7.  I am leaning toward the dual 7 because I thought with only 10 inches of vac that the second diaphragm may give it some additional surface area and help me stop.

The current one is dead, doesn't leak vacuum, but I think the back side is not working so it never "assists"  The truck sat for 14 years and its the original from 1971

[BruceS]

Ross, I've been impressed with Booster Dewey's service and quality of his work.  I would be wary of the cheap options; I had a diaphragm tear on my Galaxie and it caused a massive vacuum leak in a matter of months.  Not so with Dewey's jobs.  You could also give him a call (pacific time) and get his weigh-in on the dual vs. single question. 

Bruce

[My427stang]

Ross, I've been impressed with Booster Dewey's service and quality of his work.  I would be wary of the cheap options; I had a diaphragm tear on my Galaxie and it caused a massive vacuum leak in a matter of months.  Not so with Dewey's jobs.  You could also give him a call (pacific time) and get his weigh-in on the dual vs. single question. 

Bruce

Good advice Bruce, I will likely do that

[turbohunter]

Speaking of weighing in, that might be what I felt with the single 8. A pickup weighs quite a bit more than a little hot rod that I'm assuming is why these smaller diameter boosters came into being anyway. There may be a weight ceiling for each style.

[My427stang]

Marc, I reread your post and missed i, did you say the dual 7 did NOT behave like the single 8?  Was it better?

[turbohunter]

Ah, yes I guess it is a bit vague. I did not have any problems with the dual 7 inch. The 7 was the first small diameter I tried and it worked fine. I bought the 8 inch because I had heard a few comments on the 7s being to small to do the job so I bought the 8 to see if there was any difference (I should have bought a dual 8 so I could compare apple to apples). There is really no difference that I could feel save for that "feeling" I described. I do not remember feeling that with the 7 inch dual (a few years ago). It's also possible that since I drive the truck more now, I am more in tune with how it feels. So the upshot is that they do work. And I don't know how much my weight comment would apply because it's really your mechanical system that deals with that not help with the pedal that a booster gives you.
I'm curious, are you thinking about this for your 4x?

[My427stang]

Yes, however I found a few companies that make a dual 8 with a bracket, adapter, and a GM master made directly for 67-72.

What I am waiting for now is to see the actual OD of an 8 inch dual.  My 8.75 inch stocker was very close to the Lemans V/C so if the dual 8 inch is slightly smaller, I will go that route instead of the dual 7.

I haven't been in a situation where the truck wouldn't stop, but it's a little too fast to be messing around and eventually I want to put discs on the front, so figure may as well replace the bad booster with something more

[turbohunter]

I just went out and measured my 8 and its about 7.5 to 7.75 so you should pick up some good relief.
Here's a pic so you get an image (kind of).

[chris401]

Concerning stock boosters, I used a 71 brake booster my 65 F100. It cleared the ribbed M/T HP 427 valve covers on the 390. If you have the thin light weight 428 valve covers you will have even more room due to the more rounded edges.

I think your 71 has the same small rear drums used in 65. My transplant was from a 73 F150. I remember the rear drums being 1/2" wider. The booster had no problem keeping power assist on this set up.

[My427stang]

Concerning stock boosters, I used a 71 brake booster my 65 F100. It cleared the ribbed M/T HP 427 valve covers on the 390. If you have the thin light weight 428 valve covers you will have even more room due to the more rounded edges.

I think your 71 has the same small rear drums used in 65. My transplant was from a 73 F150. I remember the rear drums being 1/2" wider. The booster had no problem keeping power assist on this set up.

My 71 booster fits fine with cast alum Lemans V/C , it's just junk.  Truck sat in a garage for 14 years starting in 1995 and it never worked again.  However, it's sorta close, likely 1/4-ish inches of clearance, so figured a slightly smaller dual may help with both clearance and to offset that the 445 doesn't make a lot of vacuum, so I am looking at replacing it with something that may be a bit more forgiving.

[chris401]

the 445 doesn't make a lot of vacuum, so I am looking at replacing it with something that may be a bit more forgiving.

I had decent vacuum in those days, don't remember anything less than 14". Probably on the bottom end of low vacuum tollerance but it was good enough for DFW traffic. I can't say if it was the original 71 or if the rebuilder did anything special to it. If you went into "manual antilock" mode you would probably loose assistance by the 3rd stab.

[My427stang]

I ordered an 8 inch dual diaphragm made for the truck.  Pretty neat hot rod setup, but has bracket and extension to fit my truck and a GM drum/drum master.  Anything has to be better than what I had for performance, but we'll see in the end.  Will update likely next weekend

[My427stang]

Here is the end result, sure worked out nice.  The cam is a little stout for much more than a stomp at idle, but driving it's like a different truck, really nice

This is a dual 8 inch with a Vette 4 wheel disk master, seems to work great