37 replies to this topic

[wayno]

I put a 1978 620 master brake cylinder(dual M/C) in my 1969 datsun 521 kingcab sitting on a 81 720 diesel frame, I was able to get the fronts to bleed with a lot of effort, but the backs won't bleed at all, is there some secret to bleeding a dual M/C? I spent at least an hour trying to bleed the backs, I put a small hose on the loose bleeder valve, stuck the hose in a glass of brake fluid, and I pumped away and nothing happens, when I did the fronts, bubbles came out till they were bleed. The back brakes would lock up with the single M/C, so they do work.

[jesusno2]

bad master? you bench bleed it i assume first

[ggzilla]

Did you bleed the master before trying to bleed the rear brakes? Crack the rear brake line at the master cylinder, have someone push the pedal down halfway and tighten it back up. Do that a few times.

Then proceed to bleed the rears.

[wildmaninid]

this should have the NLSV on the frame........there is a bleeding process that includes bleeding this load sensing valve......i know its not much help that i dont have the process posted here, but my service manual is at the shop

[wayno]

It has a bleeder on the master for each line, front and back, I cannot get anything to come out of them. What do you mean bench bleed? I have had a couple 720 diesels and never had a problem bleeding the brakes. The master brake cylinder is new from baxters today, I am wondering if the rod between the booster and the master could be to short, I adjusted it so it just ingauges when I bolt the master to the booster.

[ggzilla]

"Bench bleed" is nother way of saying bleed it at the master cylinder. You can also do it before you install it (on the workbench).

[ggzilla]

It has a bleeder on the master for each line, front and back, I cannot get anything to come out of them.

The master cylinder is defective. This is an increasing problem with the quality of new cylinders, now that they aren't being made in Japan...

[wayno]

this should have the NLSV on the frame........there is a bleeding process that includes bleeding this load sensing valve......i know its not much help that i dont have the process posted here, but my service manual is at the shop

This was the last thing I tried before giving up for the night, there are just so many differant trucks that make up this vehicle, the NLSV is out of a 85 720, the booster is out of a ford courrier, the brake master cly.(new today) is out of a 78 620, the frame and brakes are a 82 datsun diesel, it worked fine with the stock 1969 datsun single M/C on the booster.

[wayno]

The master cylinder is defective. This is an increasing problem with the quality of new cylinders, now that they aren't being made in Japan...

So your saying it is no good, when I loosen the line on the bottom of the master going to the back, fluid does drip out, but it doesn't squirt out. I thought baxters sold good products, I can get the stuff cheaper elsewhere, but I wanted it to work.

[flatcat19]

I thought baxters sold good products

They do.
AutoMeter. MSD. Odyssey Batteries. FlexALite. Earl's lines. Holley. Edlebrock. Etc.
I wouldn't bother with their hard parts.

Baxter's is good for their performance products.

Which brand of MC did you get?

Through my time pushing a wrench, I've mainly focused on brakes and steering/suspension.
When it comes to brake parts it's NAPA or a brand called Centric.
Just don't buy NAPAs cheapest. And you'll have to ask to find the Centric stuff around PDX.
Never been burned by Centric. (FAST in Portland is a huge supplier of Centric.)

[mklotz70]

As I understand it, there is actually a bit of difference between bench bleeding and bleeding the m/c with the pedal. When you bench bleed, the m/c is level(which it's not in the vehicle) and you bottom out the pistons in the m/c. The pedal will not bottom them out. You can take the push rod out and use a long screwdriver to do it in the vehicle, but I'm not sure if that works with a booster and the m/c is not level so there's a small chance it can trap air in the cylinder. With that said, you should still be getting some kind of pressure. When you said you had it adjusted so that it was engaged when mounted....there needs to be a tiny amount of play or a tiny bit of tension. You don't want it pushing the pistons in at all when the pedal is disengaged. If you're bleeding it yourself with a hose and jar, the bleeder valve or line nut can't be too loose or it will suck air back in around the threads.

On that m/c, the rear circuit is toward the front of the truck. It sounds like the back(front circuit) piston is moving just fine, but I wonder if the front(back circuit) piston is moving. There's a screw on the bottom that has a pin that sticks up into the m/c bore to keep the piston from moving too far toward the pedal and interfering with the front circuit. I wonder if that's in the correct place on the piston or if it's stopping the piston from moving forward when you press on the pedal.

[wayno]

Well, I pulled the M/C off the truck and put it in a vise, put fluid in it and used a screwdriver for a brake pedal and couldn't get anything but a fine mist to come out of the rear circut, what a POS, I am off to get another one, be back soon.

[wayno]

Well I went and purchased another one this morning, differant store but it turned out to be the same brand, this one didn't work either, the front circut would bleed after I bottomed the plunger out, but the rear circut worked fine, I am giving up and putting the stock single res./circut M/C back on. I don't beleave I had a bowl of stupid this morning, I don't beleave this is my issue, but I have never dealt with dual circut M/C before except stock ones which I have never had trouble bleeding the system before.

[yello620]

You will not get anything out of them(problem you are having), until you have all of the air bleed out of the cylinder. That is the purpose of bench bleeding them.

Use the screwdriver in the master as you were doing and put a finger(lightly) over each ouput hole. Now push the screwdriver in and lightly release your fingers. Re-seal the holes with fingers and then release screwdriver. Do this repeatedly until both halves of the master have fluid pressure. They will not get pressure at the same time. You will eventually get a hard pedal(screwdriver) that barely moves when you apply enough finger pressure over the holes.

You can also plug the brake line holes and just pump the screwdriver until you get all the air out. It will bubble up into the resevoir slowly. Also tapping lightly on the body of the master helps get some of the trapped out to bubble out.

It usually takes a combination of both of these methods to get all the air out.


Now, since you already installed this thing once and have pumped a lot of air into the system because the master was not bench bleed, you are going to fight that part of the bleeding process also.
At this point i would recommend separating the systems for bleeding purposes. Go to the auto parts store and get two plugs to install in the master cylinder, and get it bench bleed with them installed.

Install master cylinder into vehicle, but do not attach both brake lines yet. Install one line and bleed that half of the system until you have a good pedal, then isntall the other brake line to the master cylinder. Now bleed the rest of the system.

Just let them gravity bleed. When you have good fluid to both sides(L to R) with the system separated F to R, close the bleeders and pump the pedal a few times. Then go back and re-open the bleeders. One at a time, Never open two bleeders at the same time. The pumping of the pedal with bleeders closed just helps interrupt the trapped air and make it go to the top(bleeder).

All of this can be done with one person, there is no reason to sit in the truck and pump the pedal. Pumping the pedal just ruins the master cylinders. People wonder why their master goes bad right after they use the pump method to bleed them.

[wayno]

I gave up, I did bench bleed the second M/C, I put the little fittings on the M/C outlets, then the little hoses on the fittings, I filled the reservors, put a glass with brake fluid on the bench, put the hoses into the glass, and then pumped the M/C with the screwdriver. I was able to get both sides to work, but the front circut wouldn't work properly, the back circut worked fine, but the front circut didn't funtion till after the rear, I had to bottom the screwdriver out to get the front to work at all, by that depth the rear had already emptyed the bore. I still have the original M/C that I was unable to get the rear to work, I'll go out and try again with it Yello620. Thankyou for you help. I already put it back together with the single circut M/C, bleed the brakes(took about 5 minutes by myself), and drove the truck around, it is better than it was before.

[wayno]

I just spent over half an hour messing with the original M/C on the bench, the front circut works great as it did before, I can empty the front circut reservoir several times, and the rear circut reservoir never changes, fact is the working throw for the rear is only the last half an inch of so, if it is 3/4 of an inch I would be surprised, you can watch the fluid moving back and forth, but it doesn't pull any from the reservoir. So then I tried pushing in the plunger, then releasing the air, cover with finger, and release plunger, it sucks against my finger for a while, but no fluid really comes out when pushing and the rear circut reservoir doesn't drop while I wait for the vacumn to release my finger tip. I think it is defective.

[yello620]

I gave up, I did bench bleed the second M/C, I put the little fittings on the M/C outlets, then the little hoses on the fittings, I filled the reservors, put a glass with brake fluid on the bench, put the hoses into the glass, and then pumped the M/C with the screwdriver. I was able to get both sides to work, but the front circut wouldn't work properly, the back circut worked fine, but the front circut didn't funtion till after the rear, I had to bottom the screwdriver out to get the front to work at all, by that depth the rear had already emptyed the bore. I still have the original M/C that I was unable to get the rear to work, I'll go out and try again with it Yello620. Thankyou for you help. I already put it back together with the single circut M/C, bleed the brakes(took about 5 minutes by myself), and drove the truck around, it is better than it was before.

Wayne, you will notice that i never ever mentioned bleeding the master into a little jar with the little hoses. There is a reason for that, it doesnt work worth a damn.

While bench bleeding, the two circuits will not build pressure at the same time. This is the system design. The rear brakes always apply first, then the front brakes. This creates some directional stability in the vehicle when braking.

Are you working with NEW or Re-manufactured master cylinders?

[wayno]

They are re-manufactured from someplace called porta, or something like that, I have decided to find out what a new centric will cost this week sometime, the first re-man I couldn't get the rear circut to bleed, the second re-man I was unable to get the front circut to bleed. I put the old one back on and was done in ten minutes. I have bought centric in the past and have never had a problem. It makes me crazy when I can't figure something out, I spent the half the day trying to get this to work, what a wast of time.

[ggzilla]

I find brake bleeding to be very frustrating.

I will never rebuild another cylinder, or buy another re-man. Only new for me. Life is too short to mess with that stuff. But I'll fiddle with other types of parts all day, it doesn't bother me. I'm fitting an intermittent wiper switch to my older Datsun now, getting connectors, tracing wires, etc.

[yello620]

Well, remans are your first problem. Absolute crap. Try calling FAST undercar in portland. Phone # 5zero3-2three6-FAST(3278). However, i am not sure if they sell to the public, if not, let me know, i buy there often enough.