Master Cylinder Push Rod.

[YukonJack]

I'm going to replace the stock master cylinder on my 47 Plymouth. I have disc brakes on the front and stock rear. I intend to remove the guts from the stock master and run a longer push rod to a master cylinder behind the stock one. I know several people have already done this. My question is what did you use for the longer push rod? I saw someone had used "all thread" steel rod, but I'm not sure if that would be strong enough. I'm afraid it could bend under pressure. 

[Sam Buchanan]

When exploring this installation I fabricated a pushrod made of steel tube with a nut welded on one end and a bolt on the other. This allowed using the stock pushrod ends with adjustability. If going this route be sure you use jam nuts on each end to prevent the pushrod from rotating and unscrewing one of the ends.

 

Edited March 4, 2020 by Sam Buchanan

[50mech]

About to tackle this one myself, thanks Sam for that extra detail. I'll try to post some specifics when I do it  this weekend ( hopefully)  as well.

Edited March 5, 2020 by 50mech

[Sam Buchanan]

The jam nut on the master end of the pushrod wasn't in place when the photo was made. The jams need to be tight....if the pushrod length changes serious brake problems could result.

[50mech]

15 minutes ago, Sam Buchanan said:

The jam nut on the master end of the pushrod wasn't in place when the photo was made. The jams need to be tight....if the pushrod length changes serious brake problems could result.

Is your new master mounted tightly against the old like I've seen photos of someone's here with a red master cylinder? If so do you know the approximate range of OAL on your new pushrod?

[Sam Buchanan]

1 hour ago, 50mech said:

Is your new master mounted tightly against the old like I've seen photos of someone's here with a red master cylinder? If so do you know the approximate range of OAL on your new pushrod?

[50mech]

On 3/4/2020 at 9:57 PM, Sam Buchanan said:

The jam nut on the master end of the pushrod wasn't in place when the photo was made. The jams need to be tight....if the pushrod length changes serious brake problems could result.

Know the thread sizes on the original pushrod off hand?

[Sam Buchanan]

5 minutes ago, 50mech said:

Know the thread sizes on the original pushrod off hand?

 

Here is the thread:

 

https://p15-d24.com/topic/50039-converting-p15-drumdrum-brakes-to-dual-chamber-master-cylinder/?tab=comments#comment-531646

 

Looks like it is 7/16-20.

[YukonJack]

Hey Sam. I just followed the link to your previous post about not being to get any pedal. Is it possible that the push rod didn't have enough travel into the new master cylinder? Just a thought.

[Sam Buchanan]

2 hours ago, YukonJack said:

Hey Sam. I just followed the link to your previous post about not being to get any pedal. Is it possible that the push rod didn't have enough travel into the new master cylinder? Just a thought.

 

I don't think that was the problem. However, since I was wanting to get brakes on the car and get it roadworthy, I eventually just chucked the whole deal and installed a new standard master.....and immediately got a good pedal. I may revisit this again when I get bored and figure out why it didn't work. It is a 1" master cylinder, don't know if that is the best size or not.

[50mech]

I've got a 1" bore Vette type going in this week or weekend so I'll have some info regarding that.

 

To my thinking an 1.125  bore dual chamber master with a hair over an inch stroke should give about a half inch travel at each piston on the wheels ( two pistons at each wheel). Where a 1" bore is dropping only to .44" travel at the wheels. 

We need far far less than that.

 

Plus remember two chambers=2 pistons. All else being equal a dual chamber is capable of displacing twice the fluid per same length of stroke. But because the pistons are hydraulically coupled the pedal effort is the same as the equivalent bore single chamber.

 

I may be proven wrong soon enough but I don't think displacement is an issue.

 

If I had to guess I would think Sam's either got some air stuck between pistons that survived the bench bleed or he just got a bum master.

 

 

Edited March 10, 2020 by 50mech

[50mech]

Ok, finished my brake installation other than an mc cover for the floorboard. I am running the original drum/drum setup.

1" bore Vette style mc.

 

I mounted the new master directly against the old master with a piece of angle iron in the last two original master bolts like another member here did.

 

Heres my pushrod started.

 

 

 

[50mech]

Can't upload any more to show it finished but you get the idea.

 

I used 3/8 threaded rod and coupling nuts. I drilled and tapped the coupling nuts halfway down for 7/16-20. ( Because good luck getting 7/16-20 rod and coupling nuts)

At the eye end I screwed it in.

At the rod end I screwed in a 7/16-20 bolt and then cut the head off to make it a stud for the rod to screw onto. 

 

Finally once I got it all cut to length I placed a piece of 3/8 galvanized pipe over the threaded rod , which had been cut to 1/16 over the length of gap between coupling nuts.

 

The threaded rod holds it all together tesion wise but the nuts/pipe stack takes all the compression force when braking....very very strong assembly. It stretches the rod very roughly ( total guess) 6".

 

I used what appear to be the same residual valves @Sam Buchanan used in his install.

 

The brakes work great.

 

I'll post full install details to it's own thread.

 

Oh, and the MC reservoir can be easily accessed. It's not under the seat.

I finally looked up whos mounting I copied. @Robert Horne

Here's his thread

https://p15-d24.com/topic/22442-master-cylinder-48-dodge/?tab=comments#comment-218520

 

Edited March 16, 2020 by 50mech

[pflaming]

I’ve posted this before, this is what I did in my ‘53 suburban.  I am going to pull the rod tomorrow and check the nuts. To get the correct new length, I put a hose over the original rod, then slowly pushed the brake, the hose slid back and I got my length. Then I welded a steel bolt on the shaft shortened the push by 1/4 of an inch. I still have the adjustable end to fine tune. Did the same on my truck eight years ago.