AW Gearheads: I Have a Master Cylinder Issue

[rp2813]

The master cylinder in the '50 GMC sprang a leak, through the pinhole in the filler cap, which in and of itself seemed odd.  I decided to remove the MC and rebuild it, as the cost of the kit was only $16 and change and it seemed like it would be an enjoyable and satisfying little project.

 

Once I had the MC on the bench, it was time to empty the reservoir of its old fluid.  Low and behold, as I was pouring out the old brake fluid, the metal inner seal from underneath the cap of a brake fluid bottle came out along with the fluid.  Well, that explained the MC problem but it was too late now; the project was underway and the fluid was so murky -- indeed, I couldn't see that cap seal floating around or I would have saved myself some trouble -- so I proceeded with the MC overhaul, re-assembled it, re-installed it, filled the reservoir with clean fluid, then got ready to start the bleeding process.

 

When pumping the brake pedal to bleed the first wheel cylinder, the pedal went straight to the floor with no resistance.  With brake pedal fully depressed, loosening the fitting at the wheel cylinder produced no fluid.  I went further and removed the fitting completely and it was still dry. 

 

I don't know where the problem is now.  The truck is up on ramps (the MC is located under the floor of the cab) and I can't back it down without brakes.  My first thought is to remove the brake line from the outlet end of the MC and check for any fluid flow at all out of the MC.  I wasn't expecting results like this, and I'm not relishing the thought of having to once again disconnect the brake pedal return spring and the MC plunger rod from the linkage.

 

Any suggestions or diagnosis would be appreciated.

 

Ralph

 

[revvinkevin]

 

 

Hi Ralph, 

 

You need to / needed to "bench bleed" the master cylinder first before installing it on the truck (or while installed, but WITHOUT the lines connected.)

 

To bench bleed the MC, you need a kit which includes plastic, threaded adaptors and a couple rubber tubes.   The adaptor screws in where the brake line would, then the rubber fit on the adaptor.   You then put the other end of the rubber tube down IN the MC reservoir (there's also a clip to hold it) and then pump the MC until no more air can be seen coming out of the tube.   Once that's done, proceed with bleeding the brakes at each wheel as normal.

 

KEEP IN MIND that in the process of trying to bleed the system at the wheel you MAY have put more air in the brake lines.

 

I hope this helps!   BTW, you SHOULD be able to get a MC bleeder kit at most any auto parts store.

 

I would have called, but I don't have your number.

 

Kevin

 

 

P.S.... everyone, and I mean EVERYONE should flush, not just bleed, but flush they brake system in their car, truck, van, SUV, etc, every TWO years.   Brake fluid is hygroscopic, which means it absorbs moisture and other contaminants in the brake system AND can cause rust in the system over time (why the fluid looked dark and cloudy).   If everyone did this every 2 years, then (in my opinion) brake system parts (Master cylinders, wheel cylinders and brake calipers) would last a LOT longer then they normally do.

 

 

[rp2813]

Thanks Kevin

In all the reading I did before starting this process, nothing was mentioned about bench bleeding.  Perhaps it's assumed knowledge, but I've never had to replace a MC before so it's all new to me.

 

I'll look into a bleeding kit.  It makes sense to bleed the MC first.  I'll contact you if I need further advice.

 

[goatfarmer]

You can also bench bleed a MC by placing your finger over the hole where the brake line attaches. Being a 50 GMC, I'm assuming it's a single system.

Anyway hold the M/C steady in a vice. Fill it with fluid, place your finger over the brake line hole, push on the piston until it stops, allowing air and fluid to escape past your finger. When the piston stops, tighten your finger over the hole, so no air can enter, and let the piston return.

A bit messier this way, but works in an emergency.

 

[hydralique]

Vacuum Bleeder Kit . . .

Master cylinders can be a PITA to initially bleed, as I discovered years ago with my first Fiat after having grown up with Citroens that don't use master cylinders. I highly recommend investing in a vacuum bleeder kit if you have a hard to bleed master cylinder. These consist of a hand operated vacuum pump with various fittings and a small clear jar that connects to the bleeder's lines. You can use it to help bench bleed the master cylinder first if you wish. After the master cylinder is installed, connect the vacuum pump to the brake bleed valve (start with the driver's side front brake) through the tubing and pull some vacuum. When you then pump the brake pedal the vacuum bleeder helps pull the fluid through the lines and into the clear jar - once you see fluid in that you know you're getting success.

I've had a Mityvac Silverline vacuum bleeder (this is the metal bodied one, but plastic is available too) for well over 20 years and have used it for many automotive tasks. It's great when changing fluid as you can suck it right from the brake resevoir, or it can be used to apply vacuum to pretty much anything. With the Silverline you get an integral vacuum guage that's handy if you need to know how much vacuum you're holding (such as for testing a vacuum advance distributor) but the cheaper plastic Mityvac kit should work just as well for brake use.

Link is to the Mityvac Silverline.

http://www.autozone.com/autozone/accessories/Mityvac-Silverline-elite-vacuum-pump-test-kit/_/N-25j2?counter=0&filterByKeyWord=mityvac&fromString=search&itemIdentifier=988888_0_0_

 

[rp2813]

Success! Thanks To All!

I went to the auto parts store right after responding to Kevin's post.  I got a basic MC bleeding kit for a little under $10. 

 

Rather than deal with the annoyances associated with disconnecting the brake pedal return spring, which is in a tough spot to reach, and removing the clevis pin for the plunger, I bled the MC by disconnecting the line at the outlet and running the tubing back into the reservoir.   That system worked perfectly.  This was an instance where having the MC located under the floor not far from the brake pedal turned out to be very convenient.

 

I followed up by bleeding each wheel cylinder and everything looked and functioned as it should.  I took the truck out for a spin on a route that would involve numerous stop signs.  It's back to normal, or better than normal considering the MC has been rebuilt and murky fluid replaced with new clean stuff.

 

Thanks to all for your input.  I knew if I posted my issue here that I'd get good advice.  Meanwhile, on the vintage GMC site my inquiry is still waiting for a response.  I guess I'll go post an update there too.

 

 

[washernoob]

Yay!

Hello there, Fellow weekend-master-cylinder fixer!

This whole weekend was consumed in replacing my 1967 VW Beetles Master cylinder, and brake hoses/lines. Also wheel cylinders and drum brake pads. FUN!

Probably not as fun as working on a 1950 GMC though.