Brake booster recommendations needed 12/70 40 series

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Kringle

Kringle

Joined
Dec 18, 2021
Threads
34
Messages
115
Location
SoCal - San Berdoo
Not positive, but I'm pretty sure the booster is no longer doing it's thing.

Brake pedal pumps up nice but as soon as I start the engine, the brake pedal goes all the way to the floor. Have bled the brake system over and over. All good fluid and no air in the system.

Anyone have recommendations of where or where not to get a replacement?

December 1970 build - photo of current setup included

20260319_175839.webp
 
Last edited:
Kringle said:
Brake pedal pumps up nice but as soon as I start the engine, the brake pedal goes all the way to the floor.
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As I understand brake boosters. the pedal is supposed to sink a little bit when the engine is running and they're getting vacuum. Is this something new for your rig? I wonder if you can adjust your pedal higher or if you need a longer pushrod.
For a new booster I went with City Racer...
 
Pighead said:
As I understand brake boosters. the pedal is supposed to sink a little bit when the engine is running and they're getting vacuum. Is this something new for your rig? I wonder if you can adjust your pedal higher or if you need a longer pushrod.
For a new booster I went with City Racer...
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It does sink a bit and then a couple seconds later goes to the floor. It didn't used to do that, so I'm assuming the booster has failed. It's a 350 conversion truck. I don't think the booster that is in there is the original style based on the replacements I have found. The shape appears like the city racer 79-83 pickup. I don't know enough to understand if there is more to it than just that.
www.cityracerllc.com

Brake Booster for '79 to '83 Toyota 4WD Pickup

New brake booster for 1/'79 to 7/'83 Toyota 4WD Pickup (RN37, RN38, RN47, RN48). This booster is brand new. It's not a rebuilt, not a reman, and there is no core charge.OE quality seals, diaphragms and check valve. Painted finish and zinc plated hardware. Direct replacement of OE part #...
www.cityracerllc.com
 
Pighead said:
To me, that sounds more like a brakes problem than a booster problem. What brakes do you have?
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disk upgrade in the front, original drum style in the back. Disk conversion was done about 2 years ago and brakes have been great. The truck sat for about 6 months and when I went to use it again I experienced the issue.

I had some very minor leaks in the back brake cylinders and thought that might have been causing the issue. so we did all new cylinders in the back and bled the brakes out more than needed. All new clear fluid coming out of the bleeders and adjusted shoes.

System pumps up nicely until the engine starts which is when the booster gets vacuum. therefore, I'm assuming it's booster related. Don't know how else the engine start would affect the pedal pressure other than the booster?

I'm kinda thinking that the guy who converted this to a 350 put a later model pickup truck booster in it. Regardless, the brakes have been great up until this "all of the sudden" issue.
 
Kringle said:
I'm kinda thinking that the guy who converted this to a 350 put a later model pickup truck booster in it.
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I'm kinda thinking that your 40 did not come from the factory with a booster at all.
How old is the master cylinder? Do you have a solid brake pedal with the engine off?
What means "system pumps up" do you have to pump the pedal?
 
Pighead said:
I'm kinda thinking that your 40 did not come from the factory with a booster at all.
How old is the master cylinder? Do you have a solid brake pedal with the engine off?
What means "system pumps up" do you have to pump the pedal?
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well, that's an interesting thought that it might not have had one new. Previous owner might have just added it during the conversion?? would make sense why it's not original style.

The brake pedal pushes to the floor when the engine is running. If you shut the engine off, and then you pump the pedal a few times and the pedal comes back up hard with the engine remaining off.

As soon as you restart the engine, the pedal sinks just a bit and then after about 3-5 seconds it sinks to the floor (under your foot pressure).

The master cylinder is a dual and was put in new by an old school landcruiser guy on this forum about 2 years ago along with the front disk brakes. By time, it's about 2 years old. By mileage it's probably a few hundred miles old.

20260320_130738.webp
 
Fire wall rib has been cut to add the booster.
Will it pump up to pressure after it is started and the pedal goes to the floor?
 
Bad diaphragm inside the booster. 1970 came non boosted. Hit up the old school 40 guy that installed it, he might know exactly which booster to minimize problems. If you liked braking prior to this, try hard to get the exact same booster.

With the rib cut, prolly any booster will fit, but you want the one that matches the depth + bolt pattern on firewall + the master cylinder.

Yours looks like a 60 series, but I’m no expert and the markings on the master cylinder are unfamiliar. Cityracer is the man if you want new booster/master.
 
I put discs on the rear and 13.3" rotors on the front and went with a double diaphragm and 1" MC from Red Line the same as City Racer. The double diaphragm has a larger diameter and you have to install the spacer behind the booster to clear the clutch MC. City Racer I heard includes the spacer. Red Line did not provide the spacer at the time I bought my brake system, but now does. The double diaphragm booster is also shorter which was good since the R2.8 is wide and I had better clearance with the new booster and MC. Red Line also adjusts the depth of the rod for a price so you don't have to buy the special tool. I think I could have used a depth mic with the right rod.

I did not have to change or extend the all thread to connect to the break pedal with the spacer between the booster and the fire wall.

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POPO AGIE said:
I put discs on the rear and 13.3" rotors on the front and went with a double diaphragm and 1" MC from Red Line the same as City Racer. The double diaphragm has a larger diameter and you have to install the spacer behind the booster to clear the clutch MC. City Racer I heard includes the spacer. Red Line did not provide the spacer at the time I bought my brake system, but now does. The double diaphragm booster is also shorter which was good since the R2.8 is wide and I had better clearance with the new booster and MC. Red Line also adjusts the depth of the rod for a price so you don't have to buy the special tool. I think I could have used a depth mic with the right rod.

I did not have to change or extend the all thread to connect to the break pedal with the spacer between the booster and the fire wall.

View attachment 4106658
View attachment 4106659
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This is great info and pix. Thanks!
 
Red Line has updated there parts and I bought from them because I was able to get everything at once when I did the big brake kit. The prop valve they originally sent me did not have M10 fittings but replaced it no charge. I found out about the spacer when I tried to install the booster. When I called Red Line about the prop valve I told them about the spacer and they said they now include the spacer.
 
peesalot said:
Thats not a 70 booster and thats not a 12/70 firewall.
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You are right about the booster for sure. It matches up with a 79-83 Toyota pickup. After looking at everything more closely, whoever installed it, crudely cut the hole through the firewall and as someone else noticed, they hacked out the rib. The vin tag definitely says 12/70.
 
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