240 Skinny 740 Booster in a 240

[SLO240]

This post may belong somewhere else. Mods please relocate accordingly if so.

This post is about fitting a skinny 740 brake booster into a 240. Please note that this may not be a complete guide and you are responsible for your own actions and decisions.

Reason for this post: I could not find anything with reasonably detailed information regarding this swap so I wanted to pass a little forward. Thank you to the community.

Why install a skinny 740/940 booster into a 240? Something different to try. Make a little more room for a 16v intake. Cannot source the stock skinny booster for the 240. Or you want to try to fit Kapelhenke.com 's fancy strut tower brace (it did not work. time for a non-ABS reservoir)

Stock Brake Booster is approximately 6.75" total depth and the master cylinder is 8.75" deep for a total of 15.5".
The stock master cylinder will not work on the skinny 740 booster because the mounting stud spacing is different and the 740 booster relies on master cylinder to seal for the vacuum unlike the stock booster which will hold a vacuum without the master cylinder installed.


Statistics:

740 Brake Booster
Part Numbers: 1387650, 532680, 13876503, 3530181, 35301811
Came Stock on '88-'90 740's, '88-'90 760's, '88-'91 780's

Diameter: 10"
Thickness/depth: ~4"

Firewall Mounting studs: (4) M8x1.25 (pitch to be confirmed)
Master Cylinder Mounting Studs: (2) M6x1.0 (pitch to be confirmed)

Firewall stud mounting Spacing: 72mm-75mm square centered on the pushrod.

740 Master Cylinder
Part Numbers: 13039020, 8602017, 6819749
Supposedly fits '83-'91 740,760, 780, '91 940

Length: ~6.325"

(3) M10x1.0 Bubble Flare Ports

Installation Process

Step 1:
Disconnect the battery and
Remove master cylinder from brake booster and push it forward. You do not have to disconnect the brake lines yet. It keeps things a little cleaner.
Step 2:
From inside the car under the steering wheel remove the pin attaching the brake pedal to the booster push rod. Remove the (4) 13mm nuts mounting the brake booster to the firewall. Three of the nuts can be reached with some extensions but the top left one is difficult and a tight fit and it took a stubby ratchet and some acrobatics.
Step 3:
If you have not made a template, now with both boosters out of the car you can make a cardboard/paper template of where to drill the holes to mount the 740 brake booster. Drill the holes and mount booster. You will probably wish to plug the original brake booster holes. I taped mine with electrical tape.






Step 4:
Massaging the strut tower. Test fit the master cylinder and note how close the front port is to the strut tower. You have to bend the brake line in a fairly tight radius. The more you clearance the strut tower the less tight the fit of the break line.






Step 5
Connect the booster pushrod to the pedal and install the reservoir. One issue I had with the pushrod and the brake pedal is that it appeared that the pushrod was about 1/4" shorter than the dual diaphragm stock 240 brake booster. This moved the brake pedal in far enough that the brake light switch was always on. The brake light switch is adjustable and you will need to do so. For now I shimmed mine with some cardboard and tape. If you have not done so already, bench bleed by using bleeder screws obtained from a Pick-n-Pull and hoses running back into the reservoir.
Step 6:
Disconnect the brake lines from the original master cylinder and straighten and bend them to get them routed to the new port locations.
Step 7: Bleed the brakes and check everything for leaks.
Step 8: Enjoy the booster and master cylinder combo that is a total of about 5" shorter.


Some comparison photos:







Comments and edit suggestions are welcomed.

Cheers,

 

[mblue240]

Thanks for all the pics. I had a skinny booster sitting on the shelf for awhile waiting to go into my 240, I sold it to pay bills before I got around to installing it though. I hadn't even looked at the strut tower though, makes me rethink it a little.

 

[AJS]

Me too. Wondering how much would be needed on a RHD thought but I am not going to do it if i need to clearance.
Good timing to as I am doing a brake service.

 

[SLO240]

There are two ports on the left side of the master cylinder. I believe that no clearencing would be required for a right hand drive.

 

[Valve]

Great info! Did the brake pedal travel change much or does it require more force for the same amount of braking? Cheers.

 

[Wren]

OP: Do you have a link for that template that you made so it could be printed and used?

 

[pinguin]

I know I'm bringing this back from the dead, but since I just did this to my car, and had to design the template from scratch, I'll leave it here for anyone who's looking: https://drive.google.com/file/d/1ydEPIXRMrqcplcSgn19LlOtuGTOKCU6a/view?usp=drive_link

I couldn't get a pdf to print properly, so it's the DXF file, and anyone who needs it can tweek it in their CAD program of choice.