Disc brake swap problem

[pmschmitt]

swapped a pair of 73 disc spindles onto my 69 300. New rotors new calipers new pads new proprtioning valve ( wilwood). I did not change out the master cylinder or brake booster as it already appeared to be a disc brake MC put in place by previous owner even though he was running power drum brakes.

Here is the problem, the car stops with the disc brakes but it doesn't feel like it is putting enough pressure to the front calipers.it is actually a worse response than with the drums. I have bled and re bled the brake lines, pedal stays hard does not drop and is not squishy, no leaks anywhere.

Where should I go from here ? Replace booster? Replace MC? Is the length of the brake piston or bore diameter different on disc vs drum cars?

All help appreciated. Pictures for reference attached.

 

[twostick]

Get a disc brake booster. Dual diaphragm puts up more boost.

Rockauto has it for about $112.

1969 CHRYSLER 300 7.2L 440cid V8 Power Brake Booster | RockAuto

Edit

You do have the rear reservoir connected to the front brakes? The larger reservoir is for the front brakes.

It looks like both front and rear lines from the MC are running thru the prop valve. On my 66 swap the only line connected to the prop valve was for the rear brakes, 1 line in from the MC to the PV, 1 line out from PV to the rear brakes.


Kevin

 

[cantflip]

You are in a scenario that has concerned me.

Now that you're neck deep, lets start picking it apart.

First thing, adjust and burnish your new brakes. Caliper slides should be free, you just put them together. Loosen the parking brake cable adjustment so the cables are pretty slack. Adjust the rear drums until they drag just a little. Then re adjust the parking brake. Fully de adjust your prop valve so full pressure is to the rears. Drive the car slowly on low traffic roads for a couple miles bring it to 30 and slow it several times... or for a shorter drive drag the brakes with one foot and gas the other, run about 10 for about half a mile. The point of either method is to get the brakes hot enough to out gas impurities, but not so hot that you are blueing the brake rotors. The scrubbing will also wear mate the friction surfaces.

Give the car a couple of good stops at low speeds, and if it feels good progressively higher speeds. Then, if it feels good you should start resetting the prop valve according to any directions that came with it. If no directions I would experiment to find where I felt less braking force and give it back a little. Sorry, I have no reference for how many turns any of that would be.

If you never get to acceptable braking, stop adjusting and driving. You will now need to go to a brake part manufacturer's web site and find their bore dimension specs for the master cyl of both applications (your car and donor car of all parts used), you also need rear wheel cylinders and both front rotors and wheel cylinders... now you can see if there was a diameter change. All parts must be able to work as originally designed... bore diameters are how hydraulic pressures are multiplied.

Also look at the size of your disc rotors compared to the size of the drums... this is not a true measurement of friction surface, but you don't ever want to go smaller than what you started with.

If you provide detailed information, maybe somebody else can accurately tell if you got a component mismatched. If things get complicated, you may need to get casting numbers from the components.

Sorry you're have troubles.

 

[twostick]

I just went to the Wilwood site and that is a combo valve so it is likely plumbed OK if you followed the instructions.

The pic shows a drum type booster so you need to swap to a disc booster that has more boost.

That should make it feel like it has some brakes.

Cantflip's bedding in procedure is spot on.

Kevin

 

[pmschmitt]

Thank you two stick and can't flip.

Two stick: yes larger reservoir is going to front and smaller to rear. Wilwood has a proportioning and metering block combination I went with since the old one was junk.

Can't flip: thank you for your input. The car is 100% 1973 Newport from proportioning valve to rotors. I have not adjusted the rear brakes at all during this project and will address that.

Rear brakes are still 11 inch drums original to the 69 Chrysler 300.

I guess if the adjustment doesn't work and the braking procedure you have illustrated .. The next step is ordering a 73 MC and disc brake booster.

Any other thoughts out there?

 

[cantflip]

A minute on rock auto I found the rear wheel cylinder size is 15/16" for both applications... good and easy. the master cylinder size you are currently using should be 1" and if your 69 came with discs would have been 1 1/8" . The 73 newport would have used a 1 1/32" bore size master cylinder.

That basically means you will have a bit less pressure making ability with the 69 disc MC vs the 73 disc MC. If the 73 MC was a bolt up arrangement I would send you there... but I doubt that's the case. You will lose a little force multiplication using the 69 disc MC... there is an internet full of calculators to help you decide what the actual loss is. I'm not the one for that part.

With 69 power booster and MC and 73 brakes you will be sub 1973 stopping force by a number you might be able to live with. This is your time to either math it out or just go with trial and error and see if you like it. Now you should be able to find a casting number guide/application guide that may help you find if that MC casting (69) was ever used with a smaller bore... then you would have something that matches.
BTW the rear drums in 69 were 11x2 1/2 or 11x3... I didnt look at the 73, but that would be a consideration to matching friction surfaces for braking effect.

Just read my own words... I am not trying to be a dick... this stuff really needs to be matched...

 

[twostick]

The drum booster is not helping the cause. The dual diaphragm booster will put in enough extra input pressure that you can compensate for the MC piston size with your foot.

A 73 MC is a 4 bolt iron unit so it will bolt to a 69 disc booster. Look to a manual disc application in those years for the smallest MC piston sizes. I'm thinking 72 up 4 bolt D100 manual disc MC would be the smallest but really with a proper booster, any C-body disc MC from 69-73 should make you happy.

Kevin

 

[pmschmitt]

Dual diaphragm brake booster ordered for car with front disc and rear drum including MC . I will let y'all know how it turns out

 

[kenmdale]

if the one in the picture is the one you ordered , you will likely end up with something different. I went through this a little while ago. It seemed everywhere I checked that used that photo, none of them could actually get that model. In my case they sent me something that didnt fit my car

 

[saylor]

did you remove any stock combo or prop valves when you put the new poomlim unit?

just making sure you didn't plumb 1 combo valve up to another combo valve...

 

[pmschmitt]

Yes removed old proportioning valve before putting in wilwood proportioning valve.

As for the power booster....that would explain why it has not shipped yet from oreilly. Rock auto said it did not have one.

How did you eventually solve your problem kenmdale? Did you find the right booster?

 

[kenmdale]

I ended up finding a used one, or you could try TheRamMan, or Booster Dewey

 

[pmschmitt]

Oreilly order arrived today. Part matched picture and application. Once I get it installed an bled we will see if the booster is the problem.

 

[kenmdale]

That does not appear to be the same as the one that was shown earlier in this thread

The one pictured earlier in this thread is the one I was after, the one on the left in your picture is the they sent that did not fit. I have a 440 with HP manifolds and it wouldn't fit onto the studs

Ken

 

[pmschmitt]

Booster fit perfectly but I had to make some new brake lines from the MC to the distribution block. Of course I stripped the new master cylinder rear reservoir threads while tightening the brake line . A new one is supposed to be here tomorrow before 2pm as most of the information I looked up on tapping the threads for a report sounded sketchy at best. Sometimes I think if it weren't for bad luck I'd have none at all. Ready to complete this project and get back on the road.

 

[pmschmitt]

Ok so ended up re using the 1inch bore diameter master cylinder that was In the car before the disc swap to ensure enough pressure and because I really didn't get the part here this weekend. The old mc had some rusty fluid and crap built up in it so I flushed it out with parts cleaner and compressed air prior to re installing. It worked well prior to this.

Here it gets interesting. Adjusted rear brakes as 2 sticks recommended and took on short test drive. Brake booster works awesome plenty of stopping power. When I did my 30 mph hard stop I locked up the brakes all the way around but the rears did not unlock. I was not far from house so I limped it home.

Jacked it up re bled rear brake lines and there was a lot of pressure still in line no air. I re adjusted brakes for zero drag and tested system again on jacks. And again the rear brakes locked up and would not release until I relieved the bleeder valve. The brake pedal is also a little sluggish to return to normal location.

Is this the master cylinder creating a one way valve? The front brakes stop hard and release normally. I get the same result with proportioning valve at full rear brake and at minimuim rear brake.

All help appreciated

 

[pmschmitt]

Could it be my push rod is not dis engaging? But wouldn't that also lock up the front brakes?

 

[cantflip]

Ok so ended up re using the 1inch bore diameter master cylinder that was In the car before the disc swap to ensure enough pressure and because I really didn't get the part here this weekend. The old mc had some rusty fluid and crap built up in it so I flushed it out with parts cleaner and compressed air prior to re installing. It worked well prior to this.

Here it gets interesting. Adjusted rear brakes as 2 sticks recommended and took on short test drive. Brake booster works awesome plenty of stopping power. When I did my 30 mph hard stop I locked up the brakes all the way around but the rears did not unlock. I was not far from house so I limped it home.

Jacked it up re bled rear brake lines and there was a lot of pressure still in line no air. I re adjusted brakes for zero drag and tested system again on jacks. And again the rear brakes locked up and would not release until I relieved the bleeder valve. The brake pedal is also a little sluggish to return to normal location.

Is this the master cylinder creating a one way valve? The front brakes stop hard and release normally. I get the same result with proportioning valve at full rear brake and at minimuim rear brake.

All help appreciated

The cleaning solvents you used have likely started to swell the rubber components... the only solvents I would use offhand are brake fluid or water... Hopefully your contamination was isolated to the master cylinder. In the case of contamination the recommendation is to replace everything. Products like brake clean and other solvents are for OUTSIDE the brake hydraulic system, never inside.

Check the cap gasket for swelling... If it has swollen, your system has been contaminated and the calipers and wheel cylinders should also be rebuilt with new rubber components. If you find rebuild kits, they are pretty cheap to do. the master cylinder may as well come apart to inspect it... if I am right, the seals will have already changed size and may have broken off pieces that could cause you trouble at the compensating ports. (which is where I think your pressure holding blockage is).

Look, don't panic... if the issue is just the master cylinder seals, you might not be that bad off, you did blow it out good with air. I just cant help but do the "by the book" thing when posting this stuff online... I don't want anyone misunderstanding and getting hurt. If the cap gasket has become too big to fit right... please don't chance it.

 

[pmschmitt]

Thank you for your help. I actually only used water to wash out MC ,after thinking about it, and scraped the rust and muck with a wire brush. I repeated this several times then blew it dry with compressed air. Sorry I said parts cleaner in initial post but that was incorrect as I had just run out of it on a previous project. I still think it sounds like MC need inspection and will start there. I appreciate all help and take no offense to anyone's input. This is not what I found for a living but I enjoy working on these old cars... I am apparently just not very good at it

 

[thrashingcows]

Could also be the rear rubber brake line is damaged and is creating a check valve situation. I have had this happen to me on a new rubber brake line that was less then a year old.