CUCV Brakes Question (M1009)

[RickTheEngineer]

Howdy! Another day, another thing worth fixin' right? Alright so I had something strange happen to my truck today. I was at a red light, foot on the brake pedal, and it was holding pressure just fine, then suddenly fell like an inch (while maintaining same pressure on pedal) then held for a while, then fell all the way. Now here is the thing, I have replaced several master cylinders in my time, but none of them gave out like this. It just kind of falls in strange increments instead of slowly sinks. This isn't the hydroboost is it? Also, other details were 3 months ago I swapped over to DOT 5 because when I bought it, it seemed to have DOT 3. I flushed out the brakes as best I could with several bottles of the stuff. The fluid was nice and purple from the lines and in the cylinder. Today when I opened the reservoir, I saw it was all brown, with a thin film of clear liquid on the top. Also no leaks anywhere to be seen. Bled the brakes, same story. Do you all think that its time for a new master cylinder? If so, would you trust a NOS part, or are the seals gonna be worn out and dry (gonna buy a K30 Master)? Thanks!

EDIT - also I presume the M1008 and M1009 have the same master part #?

 

[KLRBILLY]

If your brakes are holding I would think it could be an issue with the hydroboost.
Have you checked your power steering fluid? I had to replace my hydroboost a few months ago. It was leaking from the back onto the floorboard. Just my two cents. Good luck.


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[cucvrus]

I am thinking hydroboost. And NO the M1009 and the M1008 do NOT have the same master cylinder. And technically they do NOT have the same spacer on the fire wall for the hydroboost. If it were mine I would replace the rear wheel cylinders and the front brake calipers. Because if you have contaminated fluid in the reservoir you surely have it through out the entire system. This is where a Mighty Vac pump comes in handy to vacuum out the entire system once you have all the lines open. And while you are at it put a new rear axle hose and new front rubber hoses. Doing it right the first time may cost a bit more. I learned doing it right the first time pays off in the long run. Good Luck. I buy my parts at Autozone. I do on every vehicle I work on. Can't beat the warranty and price. I seldom have any issues with any of my parts or trucks. People on here that know me and met me know I have nice trucks. No jerry rigging and shoddy work going on here. I do my best to do it right. Take care and good luck. I hope everything works out for you.

 

[RickTheEngineer]

Well, here is the thing though, the brakes hold at first, but it does start to fall and sink given enough time (like 15 seconds) but it happens in increments. This is what I'm not sure about. Also, I though the TM said the same part numbers? And also I heard that the '84 K30 MC works, but maybe that's wrong? I also was told by some local brake shops to flush the heck out of the system at the very least. Fluid levels in the power steering up are great and clean. TM says that a failing hydroboost makes it hard to press the pedal?

 

[cucvrus]

M1009 and M1008 do NOT have the same master cylinder. The hydro boost may be the same maybe. But the mounting adapter plate is different. I do remember that much. Do as you want. But flushing out a 30 year old brake system would just be the lines. The moving parts wheel cylinders/calipers have seen a lot of idle time. Time to renew. Or Not. That was just my opinion. Do as the brake shops tell you. I was telling you what I do. Have a great day.

 

[RickTheEngineer]

I probably will do a rebuild on all the brakes and do it right, no worries there, although for some reasons the shops I call don't seem to care much haha. I want to pick up the civvy version of the master since it seems to be a perfect fit (K5 Model, not K30) since it has the larger reservoir up front away form the firewall. It says up top to use DOT 3, although from what I recall you can use DOT 5 on DOT 3 seals. I just was not sure if these models change since its hydroboosted

 

[cucvrus]

Same old same old. I use dot 5. If the cover bothers you. Keep the cover marked dot 5. If you need one that bad and don't have 1 get back to me. I have one on hand used. Good Luck. Put all new bolts and slides on the calipers.

 

[Barrman]

With the engine off, the hydroboost is just a metal rod connecting your foot to the master. Try the brakes without the engine running. You will have to push the pedal at least 3 times to get the "reserve" boost pressure used up. But, if the problem still happens after that. It isn't the booster.

The TM says the M1009 has a different master and booster than the trucks. Search for a 1985 K30 with the J code 6.2 and a 1984 K5 with the J code 6.2. Maybe there is only one version still available or something.

$20.00 a piece for new calipers and $20 total for rear wheel cylinders. Doing as suggested above and replacing is a very good and cheap idea. Hoses are your choice of price. I think the best out there right now especially if a lift is in your future thoughts is the ORD braided hoses. One of those "I don't have to worry about that ever again" things.

If you have different levels of color and sediment in your master, then you for sure have mixed fluid in there. I tried the denatured alcohol flush thing on my M715 years ago. The new master I put on after the flush and swap to BFS rusted out in less than a year on the inside. The next master has lasted 5 years with no rust or issues.

I have all the hoses off my Cowdog M1007 project right now and was just a few minutes ago blowing through a hose into the lines that hook to the master and watching what came out of the ends. Black goo mostly. This is after they have sat for a week open. I don't want to spray paint eating DOT 3 around my truck, so a vacuum will be put on every line later. Might even mix in some brake cleaner spray so I know when I have gotten all the slime out.

 

[RickTheEngineer]

Yeah, at this point I am going to overhaul the entire brake system, hopefully not the lines if they are still any good. But here is the thing, I'm in a moral debate on if I want to use DOT3 or DOT5 since they both have pros and cons. It's a daily driver, and I want it to hold up to the test of time. Two questions. What is BFS? (Brake flush service?) And also why did you master rust out in only a year? Did you blow out the alcohol with compressed air? Also, the flush I'll be doing is only in the steel lines themselves since I will replace literally all brake components that isn't the booster... I'll try and check to make sure it isn't the booster later today.

 

[cucvrus]

Use a mighty vac and vacuum new fluid from front to back. It works perfect and you can bleed the brakes without any help. The mighty vac pump is about $100. and you can bleed the fuel filter without turning the engine over and check for fuel leaks and all. Very handy tool. The one I pictured is less then $50. Very handy tool.

 

[doghead]

BFS = DOT5 = brake fluid silicone, all are the same.

 

[doghead]

If you think vacuum bleeding is great, you should try a pressure bleeder.

They work even better, especially with dot5.

 

[cucvrus]

With a mighty vac pump you can pressure and vacuum. I did use pressure bleeders at the GM dealership. I just never had much use for one at my shop. I guess I never thought about having one. I have a lot of cumbersome tools that I never bother to use much anymore. But that is a great idea. I use my old shop vacuum to vacuum out master cylinders and it works well.

 

[Barrman]

I don't know why my M715 master cylinder rusted so bad. I had DOT 3 in it, vacuumed all of that through the bleeders at all 4 corners, put denatured alcohol in the master and vacuumed that all through. Then I filled it with surplus BFS that was yellow. I vacuumed it through all 4 bleeders until I had solid yellow in the vacuum cup. Less than a year later I had runaway rust inside the master cylinder. A new master and all has been good with Brake Fluid Silicone.

I love the pressure bleeder. I have either screwed a quick connect air fitting into a military master cylinder cap or welded one on so I can just change to that cap, clip it on, open the valve and then go open the bleeders with the fluid coming out.

That isn't so easy with the large cast iron GM master cylinders. I have several "adaptors" that are supposed to seal up the top and then the hose can be hooked to them. It seems to always make a mess for me. So, I just do the vacuum bit for the GM vehicles.

 

[RickTheEngineer]

One last question for you guys, since I am essentially going to do an overhaul (and do it right at that) I want to know if you could choose, would you go with dot3 after the overhaul, or stick with dot5? I will use this truck as a daily, and I live in the northeast, so moisture varies quite a bit. And I actually got a mighty vac from a buddy of mine today!

 

[cucvrus]

I live in Pennsylvania. I use DOT 5 in all my CUCV's. It works why change it now? I vacuum out the fluid on occasion and just put fresh in. I changed all my brake parts when I bought the trucks and that was the last time I ever had to do anything to them as far as brakes are concerned.

 

[Barrman]

I am doing all new parts except the metal lines on my Cowdog project. Silicone for me. I like it. Parts don't rust and seem to last a lot longer for me.

When I did all new everything on my gasser M35 back in 2007. I even made new lines. All new lines for the entire truck. A thread like this one convinced me DOT 3 was just as good and silicone wasn't worth the extra money. I had rust in the master and a stuck wheel cylinder within a year. I flushed it, redid the airpack and put in BFS. 10 years later it has no rust and good brakes. I'm sold on silicone.

Several vendors run sales on surplus bfs every so often and some pops up on this site too. Buy it by the gallon. I have seen yellow and purple. I like to rotate colors so I know I have a good flush when I remember to flush the systems. Which isn't as often as the TM says to do it.

 

[Sergeant1983]

I would stick with original dot-5. I had similar brake issue and the prevoius owner intermixed fluid. I sucked out all the fuid I could from the master, then bleed it with dot 5 till it was comming out solid purple. Then I replaced the master cylinder, calipers and front flex lines. Rebleed it puting new pads on the front. Rear brakes seemed to work fine. So opted to only do new shoes on them. Original wheel cylinders seemed to work fine. That was about a year ago and its still doing great. PS I did 5 overseas tours with the CUCVs, including 3 in the Middle East. They worked great as originally designed.

 

[RickTheEngineer]

Alright everyone, I am ready to do the job tomorrow, and I have a mightyvac, new calipers, new wheel cylinders, and MC. I have one final question for all of you. I have heard on a lot of forums out there about the use of denatured alcohol to clean the inside of the brake lines and then using low pressure compressed air to blow all the junk out and also to properly evaporate the alcohol. I am concerned if this will be harsh on the proportioning valve (the only rubber component left on the truck). Any thoughts? Also, a dealer told me to switch to DOT 5.1 and ditch the DOT 5 and 3 mess. What is the best way to get all the old fluid out of there once and for all? I was considering removing all the lines from the proportioning valve, and just using denatured alcohol in the hard lines and blow it out, but I wouldn't know how to clean the the proportioning valve.

 

[cucvrus]

Use the mighty vac and vacuum out the system till you have nice clean fluid. I resurrected these trucks from the grave and that is all I ever done. Unless you want to change all the steel lines, rubber lines and the proportioning valve. That is the only way I see it done. Don't introduce another chemical into the mix. Just take everything off and vacuum out the lines. Put everything new on and start at the right rear and then left rear. Vacuum till you get matching fluid color from the bleeder as is coming from the new container. Then up front right side first then left. Front you should be able to bleed easier then the rear. Good Luck I think you have this licked. Please report back. It is an easy job.