Master Cylinder going out
 

Well, while driving Rusty today...last minute present hauling :lolsmack2:...I pressed the brakes due to traffic coming to a stop, and the pedal sank to the floor. :ahhhh: I was able to quickly pump the brakes and get it to stop. Based on that and signs of brake fluid on the floor in the cab, I assume the master cylinder needs replacing.

So, is there a cost effective way to add a dual master cylinder...and power booster? When adding a dual MC, are there pre-made brake lines, proportioning valve, etc. that are cost effective to acquire? :headscratch:

Re: Master Cylinder going out
 

Hello APHaynes,

I can relate my experience in going from a single to dual master cylinder in my 64 GMC 1500 pickup. I can also assure you that this is a good thing to do after having the unfortunate experience of a front brake hose springing a leak, which caused a complete hydraulic brake system failure. Luckily, this model GMC pickup has an effective hand brake that allowed me to get home without running into anything.

I'll start with some background. Stopping is important, so I was nervous about taking on major brake work (this might come from experience since it never bothered me 30 years ago), so I did a lot of research before doing it myself. First, my truck has drum brakes on all four wheels, so it is not necessary to install a proportioning valve. Systems with disk brakes on the front or on all four wheels need to be treated differently. Second, I do not have power brakes. The truck came with factory power brakes, but this was removed many years ago because it gave too much assist for daily driving, and I have not had a need to replace it.

I purchased the earliest dual master cylinder that came standard on GMC pickups (I think this was in 1967), which seemed to fit fine. Installing brake lines was not so simple. To start with, the front (toward front of truck) and rear "ports" on the master cylinder have different sized openings that both require adapters to connect with standard brake line male fittings. I found these adapters at NAPA. I then started with straight 1/4" lines (already flared and with male fitting installed) purchased from NAPA and bent them to fit. FYI - the front brake line connects to the front port on the master cylinder (as best I could find out, but this could be corrected by someone who knows better).

The line to front brakes can be attached to the existing brake line on the drivers side at the same point as the line from the original master cylinder, which is easy. But then you need to isolate the front and rear brakes. I did this by running another line under the engine, disconnecting the existing line to the rear brakes and plugging it with a threaded plug at the T connection where the front and rear brake lines come together, then used a threaded coupling to connect the new and old hydraulic lines to the rear brakes (after putting a 90 degree bend at the end of the new line).

Making a new line fit under the engine was a major challenge for me, since it had been many years since I last needed to bend a brake line and am finding it harder to work under the truck. I started by making a wire pattern (beginning at the passenger's side), then ruined my first attempt by kinking the brake line. After getting a better bending tool at NAPA, that held the line on both sides while making smooth bends around a nylon roller, my second attempt fit (but this was mostly a matter of luck). Because I was worried about chafing from vibration or frame movement, I covered the line under the engine with plastic conduit to hold it away from metal parts and used nylon zip ties to fasten it to the existing brake line. I should also mention that it was not easy to work under the oil pan or to work the bent line into place. I have since learned that there are other options that might be easier, such as flexible stainless steel lines, or you might route new lines to the back brakes without as much bending.

Once the lines were in place and connected, I thought it was done. Wrong again. I could not bleed all the air out. This was not expected, and I finally resorted to limping the truck to a local repair shop where they used a pressure bleeder to solve the problem. This had the added benefit of getting the old hydraulic fluid flushed out without getting it all over the street or my driveway.

My truck now stops as good, or bad, as it used to. I have never had to lock the brakes with a load in the back, but can easily lock all four in daily driving. However, it is never going to stop like a newer car, so I am always trying to keep some extra stopping distance - which isn't always easy with small cars jumping in front on four-lane roads.

Good luck. This wasn't a complicated project, but it did take time to find the right parts and make it all fit together.

JRMunn

Re: Master Cylinder going out
 

Thanks for sharing your experience JRMunn. :thumbsup: Hopefully it will make the job a lot smoother for me once I am able to get started on it.

So from your experience it seems adding a power booster is not really necessary...correct?

Re: Master Cylinder going out
 

APHaynes,

All I can say is that I have not needed a power booster. This doesn't mean that it wouldn't work for you, and others would need to comment about how much it helps. A booster system would require a different master cylinder than I used, but this probably would not take much extra time to install because, for me, most of the time was in the bending and installing of separate front and rear brake lines.

JRMunn

Re: Master Cylinder going out
 

aphaynes,

I converted my single-reservoir master cylinder to a dual-reservoir with a power booster. I used a 1975 GMC as my donor vehicle and swapped out the rear end as well as added disc brakes up front. Everything bolted in easily as General Motors did not change frame width for many years, up until 1987 I think. Anyways, the power booster bracket used the original single reservoir bolt holes on the firewall to mount up. I drilled a couple more holes to completely bolt it to the firewall as the bracket had 4 holes in it. I used the proportioning valve since I converted to disc brakes. My truck has a much better stopping distance than before. That being said, if you are looking to just add a dual reservoir and power booster and leave your original 4 wheel drum setup, I don't think it would be too difficult to add a 1975'ish master cylinder and booster to your existing brake system. Maybe someone here in the club has done exactly what your are thinking about and can shed some light on the subject.

If you look at my profile photo album, you will see some pictures of the brake conversion I did.



John

Re: Master Cylinder going out
 

John,

Thank for sharing your experience. I hope to be tackling this task very soon. I just have a car I have to get running again first.