Brake Master Cylinder Replacement

[BrandonBDenver]

Hi I am replacing the brake master cylinder on my 1946 Plymouth (P15) Sedan. I know that you have to remove the floor panel to do this but is it possible to do by jacking the car up and removing the master cylinder from underneath? Seems I missed a small bondo patch on the floor when buying this car and removing the panel may damage the previous owners fix. Thanks! 

Edited October 17, 2021 by BrandonBDenver

[RobertKB]

It can be done as I have done it that way. Very awkward and I found it virtually impossible to put back that way. In fact I think I removed the floor panel to finish the job. I would recommend removing the panel and doing any needed repair work at the same time. 

[Dodgeb4ya]

Yes it can and was done from underneath at the dealers and brake shops.

I have also done them that way for years but now days most cannot or don't want to do it to me the much easier way.

Takes about an hour and a half start to finish including pressure bleeding for me from underneath.

You probably better remove the floor panel.?

[BrandonBDenver]

2 hours ago, RobertKB said:

It can be done as I have done it that way. Very awkward and I found it virtually impossible to put back that way. In fact I think I removed the floor panel to finish the job. I would recommend removing the panel and doing any needed repair work at the same time. 

What presents the biggest challenge when trying to reinstall?

[BrandonBDenver]

46 minutes ago, Dodgeb4ya said:

Yes it can and was done from underneath at the dealers and brake shops.

I have also done them that way for years but now days most cannot or don't want to do it to me the much easier way.

Takes about an hour and a half start to finish including pressure bleeding for me from underneath.

You probably better remove the floor panel.?

It seems easy upon inspection but everyone tells me it is very hard with the floor panel intact

[RobertKB]

27 minutes ago, BrandonBDenver said:

What presents the biggest challenge when trying to reinstall?

Everything, especially the pedals if you do not remove them which is a job in itself. Might have been easier if I had a lift but I don’t. My advice, take out the floor panel. Thus ends the sermon. 

[Dodgeb4ya]

I have done so many brake jobs on the old MoPars over the last 45 years ...second nature to me....

All the newcomer's...new things to learn and then the need to have good tooling and a good mechanical mind.

[Kilgore47]

I was able to replace the master cylinder on the P15 from under the car.  I believe that from the factory the shaft that holds the clutch and brake peddles has a roll pin that holds the shaft to the master cylinder.  That pin wasn't present on my car.  If the pin is present then you would have to remove the floor pan to get the master cylinder out.  You wouldn't be able to remove the shaft to free the peddles.  All that said - the reason I removed the MC from under the car was that I didn't know any better.  Didn't know the floor pan could be removed.  Then I joined this site and I check here first now.

 

Note:  The MC I got was a reproduced part.  The hole for the shaft was about a millionth of an inch too small for the shaft to fit through.  Found this out after I had the MC installed.  Took it back off and went to the machine shop to have it bored to the 3/4" dia it should be.  Should have checked it but I didn't.  Probably a millimeter conversion problem.  Live and learn - sometimes the hard way.

[Eneto-55]

58 minutes ago, Kilgore47 said:

I was able to replace the master cylinder on the P15 from under the car.  I believe that from the factory the shaft that holds the clutch and brake peddles has a roll pin that holds the shaft to the master cylinder.  That pin wasn't present on my car.  If the pin is present then you would have to remove the floor pan to get the master cylinder out.  You wouldn't be able to remove the shaft to free the peddles.  All that said - the reason I removed the MC from under the car was that I didn't know any better.  Didn't know the floor pan could be removed.  Then I joined this site and I check here first now.

 

Note:  The MC I got was a reproduced part.  The hole for the shaft was about a millionth of an inch too small for the shaft to fit through.  Found this out after I had the MC installed.  Took it back off and went to the machine shop to have it bored to the 3/4" dia it should be.  Should have checked it but I didn't.  Probably a millimeter conversion problem.  Live and learn - sometimes the hard way.

It was a long time ago, but I thought that I had used a press to remove this shaft from my old MC, and press it back into the new one.  (Could be a faulty memory - wouldn't be the first time, and certainly not the last, the way I'm heading....)

[Dodgeb4ya]

The shafts cannot be loose in the master cylinder.

This makes sure the pedals are not wobbly and don't scrape the floor pan openings.

I have seen the shafts pinned, bolted and pressed onto/ into the master cylinder bore.

Now days all are pressed into the cylinder using preferably a press.

[BrandonBDenver]

Thank you for all of the replies. Looks like removing the pan is the winner haha! (followed by welding metal in place of the bondo patch I discovered)

[DonaldSmith]

You'll want an access cover, for checking and filling the reservoir, and checking to see that that little hole isn't blocked.   My car and its access cover were long separated, so I used an electrical box cover.  I clipped the opposite corners off the cover, so it would be closer to the shape of the access hole.  If you are re-doing that area of the floor, you can make the hole and cover any shape.   (The PO re-did the brakes with DOT-5, so the cover reminds me of that.)

 

 

[48jumpdoors]

9 hours ago, DonaldSmith said:

You'll want an access cover, for checking and filling the reservoir, and checking to see that that little hole isn't blocked.   My car and its access cover were long separated, so I used an electrical box cover.  I clipped the opposite corners off the cover, so it would be closer to the shape of the access hole.  If you are re-doing that area of the floor, you can make the hole and cover any shape.   (The PO re-did the brakes with DOT-5, so the cover reminds me of that.)

 

 

So in this photo is it the entire larger black area that needs to come out? Do you have to take out the front seat? I have not yet taken a look at the floor board of the 48 so I dont know if I have this large of a floor  area that can come out.

[Dodgeb4ya]

It's a job removing the floor pan, rubber matt/ carpets pedals accelerator pedal etc.

If the floor is rusty the 1/4" X 20 bolts can break.

Taking the seat out will make easier removal of the floor panel too.

[DonaldSmith]

As shown in the photo  of my floor panel, my seat was in at the time.  I've removed and reinstalled the floor panel with the seat in, but after its initial stubborn removal. 

If you are trying to get the whole panel out, it is easier if you remove the seat.  If you decide to replace the master cylinder without removing the floor panel, at least  you can see that there is an access panel for the master cylinder. 

[Happy 46R]

I just finished a re + re of the MC on my 46 D25C and removing the floor panel made the job much easier and less frustrating. (No lift and just jack stands for height) As for the shaft holding the pedals in place once I removed the horseshoe clips the shaft was easy to remove with very little force applied to wooden dowel. A little bit of lube made it just as easy to put back into the new MC. I asked the question on the forum about a cloth gasket like material that was partially around the edge of the floor panel and was advised to use a thin rubber to seal the two seams. Not sure if there is an actual part or material for this application but a split bicycle inner tube was just the right width and thickness. Dave