Master Cylinder Access Doors
By Jim Clark (The Hot Rod MD)

Many early model cars used to build hot rods have the master cylinder mounted on the frame beneath the floorboards.  This makes filling or checking the brake fluid level difficult unless there is some form of access to the unit.  Some of these early vehicles have a removable access cover of some sort.  Others need an access cut into the floorboard and some kind of cover fashioned.

On some of my previous projects I have cut a hole in the floorboard and cut a cover from sheet metal that was then screwed in place.  This works but is inconvenient because the carpeting needs to be lifted and screws removed to gain access.

My ’32 Ford roadster has the master cylinder beneath the floorboards so I looked for a neater and more convenient way to provide this access.  I found a line of stainless steel hinged access doors that work great for this application.

They are available from Rock Valley Antique Auto Parts, in a choice of four sizes.  The smallest one 6” x 6” (outside dimensions) to the largest 11” x 15”.  I chose the smallest one, in a polished finish, because I have a single master cylinder and plan to leave the access door exposed through the carpet.  I will protect it from foot traffic by using a small mat over the access door area.  The 5” x 8” access door is the preferred one to use when the vehicle is equipped with a dual master cylinder.
The accompanying photos show the process that I followed during the installation.

Access Door MD-1resized

This hinged stainless steel access door from Rock Valley Antique Auto Parts offers a solution to the problem of access to master cylinders, fuel pumps and other areas requiring routine service.

Access Door MD-2resized
Access Door MD-3resized
Access Door MD-4resized

Rock Valley’s access doors are offered in four sizes.  Shown here is the 5”x8” door usually used for dual master cylinders, 6”x6” door usually used for single master cylinders, 8”x8” door usually used for access to T.P.I. pumps in the fuel tank.  The fourth size is 11”x15” and is usually used for larger items like batteries located below the floor.

Access Door MD-5resized

I needed an access door to the master cylinder that is located beneath the floorboard on my roadster.  First I measured the distance from the corner of the master cylinder to the rear of the floorboard on the underside.  Then I transferred those dimensions to the topside.

Access Door MD-6resized

My roadster is equipped with a single master cylinder so I chose this 6”x6” (outside dimensions) polished access door.  It will be installed above the carpet so the polished model was more desirable.  A small mat will be used to protect it from foot traffic in the future.

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Backside of the access door shows the twist-latch mechanism.  Recessed lip around the door perimeter creates a smooth leak-resistant edge for the door.

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Access door is used to locate the position of the opening that is to be cut into the floorboard.  An additional 1/8” needs to be added to the hole on the sides to allow for the flange on the door opening.

Access Door MD-9resized

An access hole is drilled into the floorboard to serve as a starting point for the jigsaw blade.

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Floorboard is removed from the vehicle before cutting because the jigsaw blade strikes the master cylinder when the floorboard is left in place.

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The access door is already drilled with #8 size holes around the mounting flange.  Panhead wood screws or sheet metal screws can be used to secure the access door to the floorboard.  I chose to use #10-24 x 3/4” stainless panhead screws and matching #10-24 x 9/32” stainless pronged tee-nuts to provide a permanent attachment point in the floorboard.

Access Door MD-12resized

Holes are drilled into the floorboard using the access door as a guide.  I enlarged the holes to #10 while drilling these holes.  Then the access door is removed and the holes in the floorboard enlarged to 9/32” for insertion of the tee-nuts.

Access Door MD-13resized

Tee-nuts are placed into the holes and hammered into place on the bottom side of the floorboard.  The prongs secure the threaded inserts in place.

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Access door is mounted to the floorboard utilizing the #10-24 x 3/4” stainless panhead screws.  Carpeting will fit beneath the flange when the car receives its finished upholstery.

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This 6”x 6” access door provides a 4-1/8” x 4-1/8” opening directly above the filler for the master cylinder.

Access Door MD-16resized

Using a funnel makes the job of adding fluid to the master cylinder cleaner and easier.

Gaining access to components located beneath the floorboards of a hot rod can be done by cutting a hole and covering that hole with a removable plate.  This works fine but is not very convenient when routine servicing needs to be performed.  This assortment of hinged stainless steel access doors makes the process much more convenient and provides a more finished appearance.

RESOURCES: Rock Valley Antique Auto Parts


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