Larger brake booster?

[Ozz]

Anyone know if a 8.5" diameter brake booster from a 1980 720 truck will bolt on a 1977 truck? The original booster is about 7"...looking for some additional boost. I have stock front discs with stock Ford rear drum brakes (on a 9 inch Ford rear end). My 350 V-8 powered truck just needs a little help.

 

Thanks in advance

[wayno]

1977 Datsun 620s didn't have front disc brakes, are the disc brake rotors you have vented or are they solid rotors?

Do you have the room in the engine compartment for a larger booster with that small block in there or will it hit something?

Can you post a photo of the area where your brake/clutch masters are so I can determine if you even have the room.

Edited May 3, 2020 by wayno

[datzenmike]

On 9/15/2019 at 10:42 AM, Ozz said:

My 620 got a full front clip 'way back in 1980, couldn't stand the kingpin/drum setup. A few years prior to the clip I put on it I transplanted a Chevy Vega front suspension/ brake setup...bad move because it had so much bump-steer that it was crazy to drive (lack of knowledge at the time)......  my son pulled the tired 302 out to install a 350 Chevy and THM350 trans. He went drag racing with it at that point. 

 

 

 

This is the problem with not having a project thread where all the questions can be asked. Anyone reading this has to ask all kinds of questions to figure out what you have.

 

Look for clearance problems with the steering column below it. The 720s used a long stand off pedestal but that may get in the way with any other engine.

[wayno]

8 minutes ago, datzenmike said:

Look for clearance problems with the clutch master and the steering column below it. The 720s used a long stand off pedestal but that may get in the way with any other engine.

Not the 1980 Mike, it has a short pedestal.

[datzenmike]

Yeah I was thinking the larger diameter wouldnt clear the clutch master and the steering column so the later one might be tried. It 2 or 3 times longer. It was too long for the '80 as it would hit the air filter.

[Ozz]

 

I see how questions can arise. Hey I'm over 70 years old and not really too familiar with this posting stuff so be patient with me. The truck is actually a 1973 but I did a front clip to a disc brake front end many years ago. I have room for a bigger booster and I can fab/modify anything I need to. The clutch master is gone since the GM 350 trans was fitted. The new steering column provides plenty of room. I think I figured out this photo thing now so I'll go out and take some more pictures.

[Ozz]

The rotors are not vented, about .500" thick. After drag racing for 10 years the rotors were badly scored and turned blue...had to replace them.

[wayno]

You can upgrade your front brakes from what you have(non-vented) to vented rotors, the vented rotors take a lot longer to get hot and start fading, I know this because I haul wood loads with my work truck and have had everything from front drum brakes to 1990 Nissan V6 dual piston caliper disc brakes(vented), what I am saying is drum brakes last a couple minutes going down hill, your non-vented disc brakes last a little longer, and vented brakes last quite a while(3 or 4 mile down hill grade in traffic), the later Nissan brakes are way better brakes and if you upgraded your clip to a Datsun ball joint front end the later Nissan brakes will bolt right into your truck, I literally mean it will bolt right in with no modifications. 

[wayno]

It appears you have the room for the 1980 booster but it is going to be close to the valve cover, you will need everything a true 1980 Datsun 720 has from the brake master to the brake pedal connection inside the cab, you may have to make a new pedestal to mount to the existing firewall mount pattern or drill new holes in the firewall, or maybe you will get lucky and the booster will bolt to the pedestal you have in the truck now.

I have upgraded my brakes on all my 520/521 trucks, but I had to make what I needed to fit my engine bay.

 

Edited May 3, 2020 by wayno

[Ozz]

Yes, it is a ball joint front end. Thanks for the input.

 

I will post better pictures in a short while...breakfast is ready.

 

[wayno]

By the way I have seen others use the clutch pedal as the brake pedal which would move the brake master away from the engine, but that could be dangerous until one gets used to where the new brake pedal position is.

[Ozz]

Here are a few shots I just took. The clearance is more than enough. If I can just "bolt on" a bigger booster that would be great.

 

[Stoffregen Motorsports]

Neat truck.

 

With a bit of fabrication, you can move the entire pedal assembly over to fit a larger booster. I've done this on a couple vehicles and if done right, you won't be able to tell the difference.

 

How about running a hydroboost system? Or an electric booster? Maybe the reason your booster is inadequate is that you've run out of vacuum...?

Edited May 3, 2020 by Stoffregen Motorsports

[Ozz]

Lack of vacuum, I don't think so. This engine has a stock GM truck camshaft...very mild with over 16" of vac. at idle, the check valve is doing it's job and the hose is good. I considered an electric boost or a hydraboost. The hydra is a bit messy for me so we'll see what happens in the future. The reason I considered the larger booster is because of the experience I had with my 1935 Ford. For 15 years I had just mediocre brakes. I built it with all disc brakes and used the standard Corvette booster and a 7" dual diaphram "hot rod" booster. A friend of mine borrowed the car and when he brought it back suggested I put a larger booster on soooo......I got an 8 1/2" booster and installed it. I  was amazed at how much better the car is now with the change in boost. I want to give it a shot, can't hurt anyway.

 

 

[wayno]

I have that booster on my 520 and 521 work truck, I think it works great, but I have 1986 Nissan 720 brakes on them, I am using a Ford Currier booster on my 521 turbodiesel.

It appears you are due for a brake upgrade, just go out and find a 1986 720 or early D21 Hardbody, disconnect the brake lines at the hard line, remove the tie rods, remove the upper and lower ball joints from the control arms and install it all in your truck complete exactly the way it is, then bleed the brakes and your on the road again with better brakes, you could also use the 1990 D21 hardbody 2wd truck with the V6, that one has dual piston calipers.

I just checked and your booster pedestal will not work with the 1980 Datsun 720 booster, they have different mount patterns but the pedestals appear to be about the same length, so you could maybe put the plate or make a plate and weld it to your 620 pedestal and mount the 1980 booster and adjust the pedal rod to the proper length if it needs changed at all.

Personally I would try the better brakes first, the early non-vented rotor disc brakes are alright, but the newer brakes are better, they bolt right in, and are less of a hassle, if you ever wanted to use drop spindles, they work with the newer disc brakes, they do not work with the disc brakes you have.

[NC85ST]

I had a ‘73 with a Buick 215 aluminum v-8 in it. I ran the same brake setup you have, Ford 9” out of a ‘71 Bronco and ‘79 Datsun front disc’s. I used a ‘75 Chevy Monza booster on a shortened, if I remember right, Datsun 620 pedestal, stock location. I used a 4-wheel disc Trans Am master cylinder. Never had an issue with the brakes.

[Ozz]

Ok, I think I see my problem...the booster on the truck right now is the 1973 drum brake 5 1/2" diameter booster with the 3/4" diameter master. I don't have enough fluid movement or boost travel. I am going to use the 1979 620 7" diameter booster with the matching master cylinder which is slightly bigger at 8.10". The stock 1978 620 discs are perfect and now that I have the new rotors and pads on it I know I just need to make the above changes. I have a GM booster and Corvette master on the shelf that is probably real close to that '75 Chevy Monza part that NC85ST speaks of. I can try that if necessary but I think just getting all the matching 1979 620 stuff will be fine.

 

I put new rotors on today, the old ones measured .380"...badly worn from years of drag racing. Nice new .500 rotors and premium pads are much better to begin with. The system should be 100% now.

 

I don't know how my son was stopping this thing from 10 second 130 mph runs????

[Ozz]

FIXED!! The larger 13/16" master cylinder combined with a 7" booster did the trick. The truck stops really great now.

 

I couldn't find a Datsun/Nissan booster (1980 7" type) so I adapted a "hot rod" universal booster with a couple of 1/4" adapter plates and just made sure the actuating rod and the pedal pushrod length were correct. 

 

 

[Negative Ghostrider]

Not to thread jack but I have a 73 with factory drums in front. The truck got new brakes all around right before I bought it and the pedal was very firm/stopping power sucked. I fiddled with them but couldn't make any improvements. Took it to a brake shop that's older vehicle friendly and the guy was able to make some adjustments that helped a tad but he recommended a bigger booster. Parts websites say the 75+ boosters aren't compatible but from the looks of things here it seems like it should work. Should it bolt right up to the stock location/linkage/etc. or will I need to modify? Should I do the 280zx master cylinder at the same time? Thanks! 

[datzenmike]

What I would do  is pump the brakes several times to exhaust any stored vacuum in the booster. Apply the brake and start the engine. What you should experience is the brake pedal will drop towards the floor slightly. This is normal operation.

 

If no movement make sure the vacuum hose from the intake to the booster is ok, no cracks or leaks. There is an in-line one way valve in the middle of the hose. Be sure that you can suck air in from the intake side but not the other direction. If it's in backwards you won't have any booster operation.

[Stoffregen Motorsports]

1 hour ago, Negative Ghostrider said:

Parts websites say the 75+ boosters aren't compatible but from the looks of things here it seems like it should work. Should it bolt right up to the stock location/linkage/etc. or will I need to modify? Should I do the 280zx master cylinder at the same time? Thanks! 

Compatibility issues can sometimes be as stupid as having to bend the brake lines to fit the new location. I am sure the larger booster will bolt to the old setup, because I have seen many guys do it, but having not done it myself, I can't say what actually needs to be done.

 

A larger master will also add to the braking power, but beware of disc vs drum residual valves built into the master. I believe Nissan had them installed in the outlet port of the masters and are easily installed or removed as required. Drum brakes do use residual valves. If the new master does not have one built in, you can install an inline residual valve easily. https://www.google.com/search?client=firefox-b-1-d&ei=DYXaXq6fAYaitQW496mICQ&q=drum+brake+residual+valve&oq=drum+brake+residual+valve&gs_lcp=CgZwc3ktYWIQAzICCAAyBggAEBYQHjIGCAAQFhAeOgQIABBHUKygC1jlwwtgt8cLaABwAXgCgAGrBYgBniuSAQswLjEuNC40LjMuM5gBAKABAaoBB2d3cy13aXo&sclient=psy-ab&ved=0ahUKEwju2cfFnevpAhUGUa0KHbh7CpEQ4dUDCAs&uact=5

[Negative Ghostrider]

Someone mentioned that the spacer between the booster and the MC may be different if I mix/match years as well. So I may have to find a different spacer or just get a different MC regardless to make it work, apparently. 

 

Alternatively I noticed that the modern direct replacement units for the 73-74 have a 142mm diaphragm vs the 114.3 in the oem unit (per the 74 service manual). Would that difference in itself give a noticeable improvement or would it be enough to warrant the effort? For comparison the modern replacement for a 75+ is 178mm.

[datzenmike]

First do that test. Maybe the booster is bad...... maybe the one way valve is in wrong and a new booster/master won't fix this.

 

The pedestal between booster and the firewall has only one part number for all years so..... the same.

 

 

1,200 PSI brake line pressure from a 3/4 master will stop you just the same as 1,200 PSI from a later 13/16" master. A larger master will move more fluid BUT you will have to push on the pedal harder to get the same line pressure. This is why the '73 has a 4.5" booster and the later ones are 6" because more work needs to be done. I have a '73 and a '79 FSM and the booster outputs are the same for the foot pressure on the pedals. 

 

Braking is very subjective and everyone will want it better than it is. You may be used to today's cars and trucks and in reality it's already as good as it can be. About all you can do is buy shoes with better friction material.

[Negative Ghostrider]

Thanks Mike. Points well taken. I am definitely planning to test first. I have plenty of other things on this project that will need my hard earned money so I'm not about spending on things that I don't need to. 

[Negative Ghostrider]

On 6/5/2020 at 9:53 AM, datzenmike said:

What I would do  is pump the brakes several times to exhaust any stored vacuum in the booster. Apply the brake and start the engine. What you should experience is the brake pedal will drop towards the floor slightly. This is normal operation.

 

If no movement make sure the vacuum hose from the intake to the booster is ok, no cracks or leaks. There is an in-line one way valve in the middle of the hose. Be sure that you can suck air in from the intake side but not the other direction. If it's in backwards you won't have any booster operation.

I can pump and pump and pump with the engine off and the pedal never gets fully firm like in a modern car. It only firms up a very small amount. Then when I hold it down and start the engine I can feel it softens ever so slightly but there's not much movement, more just a bit of a sponge effect. Again, not the same travel as a modern vehicle. Thoughts?