68 Master Cylinder bleed - disc brake conversion

68 Grande · Guru

68 Master Cylinder bleed - disc brake conversion

cdnpont · Canadian Poncho Superstar!

Some new masters come with a simple bleeding kit comprising of 2 clear lines, a clip to retain them in the bowls, and two cheap plastic fittings to screw into the outlets.

Princess auto or any auto parts house has them. Makes for a no fuss bleed. I actually made up two hard lines with flare nuts, with the lines curved to sit down into the bowls.

Clamp the master in a vice using one mounting ear. Fit the lines, fill the bowls within 1/2 from the top with new fluid. Use a large Phillips screwdriver to push the piston, not a flat head as it can burr the inside of the piston pushrod hole.

It takes a good push to move the piston. Keep at it, in and out until no bubbles are seen. Can take a little time. The piston must snap all the way back when returning. If it doesn't, you've got a seal out of place. If you let the piston snap back you'll sometimes get a little geyser of fluid shooting up, so keep anything painted away from the area. After done the fluid will be down a little. Leave it there.

When ready to install, cover everything painted on the car, pull the lines out of the bowls and pinch them off. Fit the cap. Take the master from the bench and carefully bring it to the car. Do not allow the bleed holes in the bowls to be uncovered by fluid (basically dont tilt it too far off horizontal). Mount the master.

When the lines are close enough to fit into the master, pull the plastic ones, and work as fast as possible to get them in place. Some fluid will drip out, but it wont pour out. Just work fast. Might be an idea to mock up the master and lines dry first to make sure she'll come together.

Sure there is more to be added,

Mike Ward MB · Addicted!

Jake, I have all the fittings you will need to bench bleed your master at your disposal. I just did mine recently. It takes a while to get all the air out.

If you are thinking of going to Dot 5 brake fluid, now would be the time. I did mine and it was an easy upgrade.

Might be a good time for a coffee night to chat about it.

-- Edited by Mike Ward MB on Tuesday 6th of August 2019 05:06:58 PM

68 Grande · Guru

I bled it on the bench similar to cdnpont suggested. I think I have the master bled and the front calipers bled. Went around the block and my pedal seems hard to push, hmm, on to the next problem........ The car will stop but I have to push hard on the pedal and I am not feeling a whole lot of travel.

66 Beau · Poncho Master!

Good topic. I'll be doing mine in the next few days. I was wondering about moving the master cylinder from the bench to the car - I've seen what brake fluid does to paint. Test fitting everything in advance with a dry master cylinder is so obvious I would likely have missed it. Trying to fit up brake lines while it drips brake fluid seems like a recipe for heartache, even if everything is covered and masked off. Great advice, thank you!

68 Grande
I found some troubleshooting advice for 'hard petal' which seems to be a common problem. There seems to be several possible causes. I found the write up in the MBM disk brake installation instructions and you can download a copy here:

https://mbmbrakes.com/content/DBK6568.pdf
The troubleshooting section starts on page 8.

I'm doing a power front disk conversion with drop spindles. I'm leaving drums on the rear but I am replacing all of the brake lines with SS and installing new wheel cylinders in the rear. I decided to do this after running out of brakes coming down a local hill that wasn't all that long and I was just coasting but riding my brakes to keep the speed down. On inspection I noticed my front brake pads were all cracked and glazed. I've never seen cracked shoes like these - circumferential cracks up to about 3/32 inch wide almost all the way though the pad. I also noticed that the wheel cylinder was leaking, but not enough to even drip. In all, it was good incentive to install disk brakes.

cdnpont · Canadian Poncho Superstar!

When I did the Wilwood conversion I also has a very hard pedal, which I lived with for some time. The car braked very well and true, but I grew tired of the leg workout. A change from the 1 1/8" bore master to a 1" made the pedal feel just about perfect. Never did check the booster function, but it worked fine since the original engine. And I'm now at 15" max idle with the 496, low, but still enough for boost it seems.

It's worth checking the booster function per Wes's suggestion of the MPM guide.

Have you installed a balance valve in the rear line?

68 Grande · Guru

Yep, found the reason for my hard pedal, my push rod is too long! I put in a short rod and bled everything and even drove around the block and the brakes were locking up. So, I unbolted the MC and the brakes released.

So, I need to adjust my short rod a bit and I think I have it licked!

68 Grande · Guru

Mark, it is the exact setup as your 67 with an adjustable proportioning valve and the inline residual valve for the rears But, I think I can finish it off now with an adjusted rod.

cdnpont · Canadian Poncho Superstar!

Glad you found it! Yep, you absolutely have to get the pushrod length exact. Been there!

68 Grande · Guru

I am pretty sure that I have the correct length of rod now and I have good brake pedal travel. The booster definitely is working, and the brakes stop true but I thought I would have more brake pressure to lock up the front brakes if I really push hard. Perhaps another bleed might be in order. It is a bit soft for my liking but I think the rod is correct length, hmmmm........

seventy2plus2 · Canadian Poncho Superstar!

Sorry, are you using drums or discs on the rear?

68 Grande · Guru

seventy2plus2 wrote:
Sorry, are you using drums or discs on the rear?

Drums in the rear with adjustable proportion valve and a inline 10 lb residual, Wildwoods upfront with 1 inch shallow master.

I also see how corrosive brake fluid is with regards to paint. Takes paint off in about 4 days or so. Terrible stuff!

seventy2plus2 · Canadian Poncho Superstar!

I've done complete brake jobs (front discs, rear drums) on two of my 1970 Cdn Pontiac's (Parisienne & 2+2) and had absolutely no problems when using stock parts. I've now seen a few different posts about people needing to replace pushrods, etc. with no end of problems when using non-stock parts.

68 Grande · Guru

Just getting it all tuned in and it is already way better than the old drums up front.

cdnpont · Canadian Poncho Superstar!

Your rear brakes are likely not doing enough work Jake. Take it out on the road and setup the rear bias. In a safe place, run the car up to about 60 klms and hit the pedal full hard to see what happens. Ultimately you need to be on the verge of rear lockup. Adjust the valve up until you reach lockup. Then take it back down 1/8 of a turn at a time until lockup goes away. Setting this up made a huge difference in braking power and feel. When the balance is right you'll know it right away. With modern grippy front tires and the Wilwoods I think it would really take a slam stop to have my fronts lock. They are very linear.

On another note; Make sure you give all the lines and fittings a close inspection for any leaks, there always seems to be at least one weeper.