75' 620 front disc brake conver. issues with fluid pressure

[hawaii_620]

75' 620. I did the HB front disc brake conversion. All the parts as listed were bought. The truck creeps to a stop, similar to simply using an e-brake to stop. Forget about trying to stop in an emergency. Recently my rear drums started to get locked up. Those were completely rebuilt at the time I did the front conversion. Through the reading and responses on older posts, I believe the problem with the rear drums have been solved. Rears were adjusted and now it works much better! Thanks datzenmike. But the truck still slows down... very poorly. I do have a new 280zx MC and 620 brake booster. So, I don't know if this makes any sense, but a friend and I were trying to figure out why the truck doesn't bite when I stomp the brakes. He drove forward and hit the brakes, and it slowed down... very poorly. Front of the truck didn't dip at all, no weight transfer. He did the same thing in reverse and it slowed down at the same rate, but the front of the truck lifted slightly. We then came to the conclusion that the rears drums are doing all the work and the front discs are not working properly. Is there any validity to this simple test and our conclusion we came up with?

 

I was also wondering if there is a difference in Banjo Bolts other than size and thread. Like in terms of the actual flow that is allowed to pass through. I'm trying to think of reasons why I could be getting a lack of pressure to operate the front calipers correctly. I know when i did the conversion, I was missing Banjo Bolts and I found it off of another set of calipers. Is it possible they aren't letting enough pressure pass?

 

Thanks in advance for any insight.

Kyle

[mklotz70]

Well...since I didn't send a set to Hawaii, I'm guessing you have Beebani's kit.

 

My first guess/assumption....look at your m/c....is there an "F" and "R" on the side of the body? The "F" is for the front circuit..."R" for the rear. If you have those backwards, you'll get a lot more flow and pressure to the rears.

 

The banjo bolts shouldn't really cause a flow problem. The "flow" of brake fluid is probably not much more than about an inch to the calipers. It's not like water in a garden hose. You're transferring pressure from the m/c to the calipers via the fluid. You know those swinging balls....when you swing one, it smacks the row of balls and the one on the other end jumps, but the ones in the middle don't really move much?? This is similar, but with fluid.

[datzenmike]

Check on the inside of the frame under the pass seat to see if you have a NLSV Nissan Load Sensing Valve. These need bleeding also and may contain trapped air.

 

All I have for the proper method for bleeding is a '78 FSM, I expect the '75 would be similar if they have one.

 

 

Keep master full at all times!!!

 

Bleed the following in order.

1/ master cylinder, fronts

2/ master cylinder, rears

3/ NLSV, front

4/ front wheels, any order

5/ rear wheels, left first

6/ NLSV, rear

7/ NLSV, center

[hawaii_620]

mklotz70 - Yeah, i got my brackets off of ebay from Beebani. I'll check my F / R setup. It sounds like a no brainer, but sometimes it's the simplest things that get over looked and mess things up. I'm also going to try and re-bleed the system again, and probably even buy another master cylinder just to try that out. I've bought parts from NAPA that were faulty right out of the box, so I can't say that the master cylinder is any different.

 

datzenmike - I don't believe I have a NLSV. I've looked under the truck and didn't see anything but those junction/distribution blocks for the lines. Would I know the NLSV if I saw it? It would have more than a line-in / line-out / bolt to frame type of setup wouldn't it. I've looked at my Haynes but they don't have any pics of it, even googled it and came up with nothing. Does anyone have a picture of what I should possibly be looking for?

 

I'm going to try and re-bleed the system again. Some told me that I should try and bleed the system with the caliper not mounted and actually squeezed in a little. I'm guessing gravity bleed, because you couldn't pump the pedal or you'd close the brake and have a hard time getting them open again I would think.

 

Thank you, both of you and I'll post any updates.

kyle

[mklotz70]

I don't see any mention of you bench bleeding the m/c. That's always the first thing to do. If you don't get the air out of the m/c, the rest of the system won't bleed right. Bench bleeding is not the same as bleeding the m/c in the truck. You can do a bench in the truck, but not with the pedal. The bench bleed bottoms out the pistons in the m/c.....which the pedal pushrod will not do. You can take the pushrod out and use a long screwdriver or rod. You'll probably need a helper. Crack the bleeder valves open on the m/c, push the m/c pistons in slowly until they bottom out....close the bleeder valves and then release the pistons.

[hawaii_620]

I bench bled the m/c initially, but might have not done it correctly. I probably didn't push the piston in all the way in. I'm going to remove and do it again, then bleed the rest of the system.

 

Thanks,

kyle

[mklotz70]

Like I said...you can do it in the rig, but it's best if the m/c is level.....so jack up the rear end a bit and use a long screwdriver to push the pistons all the way in :)

[hawaii_620]

Update - Installed a proportioning valve. I've read that people are installing it near the rear of the truck, right before the rear brake lines splits to each side. My proportioning valve is installed right after the master cylinder. I wanted it under the hood so it stays clean and no one could mess with it. Brakes feel better. A mechanic friend of mine was telling me that he thinks the front calipers are still not getting the needed pressure to fully bite, for that head jolting stop. He suggested maybe seeing if a bigger brake booster was an option. I'm going to take a look into that and see what can be done. Doesn't look like much will fit in there with out a lot of modification and fabricating a new booster bracket.

 

Any validity for looking into the bigger booster? What's the thought on that from the pros on the forum?

 

 

The never ending search for good braking continues....

[mklotz70]

I've never heard of guys putting it there...that would be a real PITA to get at! I've put it where the distribution block was on the firewall. On the 521's, the rear line starts there...not sure on a 620 since you already had a dual reservoir m/c.

 

Bigger booster will simply mean less leg effort to get the same braking. Even without a booster, you should be able to get them to lock up.

 

Did you put new pads on? Did you "season" or "bed" them in correctly? If they're semi-metallics, you may want to try organics, or ceramics.

 

Do you know if your booster is even working now? With the motor off, push the brake pedal down hard(while sitting still).....start the engine....as soon as there is eng vacuum, the pedal should drop a bit.

 

Also....I'm not sure you even need the proportioning valve since you had a dual res m/c to start with. Proportion is built into the m/c via the springs between the two pistons. I usually use one and recommend one because the weight distribution on the truck is different than on the car the m/c was designed for. I've got a combination valve from a S10(non-abs) in my wife's 521.

 

What's the bore on your 280zx m/c? What year of 280zx?

 

From what I've heard from 521 guys that have put that brake setup on, it stops like power brakes, even without a booster. Considering the size of the rotor and pads, it should be putting you through the window.....or locking the wheels up.

[hawaii_620]

Well I do have a booster and I definitely cannot lock them up. Doesn't matter how hard I push the pedal. I've tried taking the booster off (vacuum line off) and I can definitely feel a difference without the booster. So I'm going to guess and say that the booster is working.

 

I have new pads. I don't recall what type of material they are though. I don't know if I season or bed them in correctly. What do you mean by that?

 

I too also thought that about the proportioning valve. I didn't think I really needed it, but since I'm desperately trying to figure this out I thought I'd try and see if it made a difference. Maybe I'm making the problem harder because I just added new element to the puzzle.

 

The bore size on my master cylinder is 15/16. Supposedly a 78 280zx as specified from the parts list required when I bought my brackets on ebay. But I just noticed that he has 280zx and 280z on the parts list. Is there a big difference in the zx and z master cylinders?

 

I see pics of 521s and 510 with brake upgrades running with no booster. I can't put anyone through my window. You could probably balance a hot cup of coffee on your lap and be pretty safe the way it is now.

 

Through process of elimination, I think I'm going to buy a new master cylinder and see what happens. If I'm not getting enough PSI to my front brakes, I have to start with the component that is first in the process I suppose.

 

I really appreciate all the feedback and input from the forum users. Especially since I didn't even purchase the brackets from you guys. Thank you everyone!

[mklotz70]

to "bed" or "season" the pads is simply to go through the proper break in procedure. It should have said on the box or the paper work that came with the pads......get them good and warm with long, slow gradual stops. No panic stops until after a certain number of miles.

 

About the only diff between the two m/c's is the bore. you can check rockauto for any other differences.

 

Did you ever verify that the front circuit of the m/c is connected to the front lines?

 

Do you still have the stock m/c? If so, and it was working.....put that one back in and try that. That would "test" that part without having to buy another.

[wayno]

Can the wrong length rod coming off the brake pedal cause problems, to long or short? Same with the rod between the booster and M/C, this is a conversion, correct?

[thisismatt]

I prefer a hard break-in procedure, like a hard engine break-in. Hawk recommends:

 

After installing new pads make 6 to 10 stops from approximately 35 mph with moderate pressure.

Make an additional two to three hard stops from approximately 40 to 45 mph.

Do not allow the vehicle to come to a complete stop.

When completed with this process, park the vehicle and allow the brakes to cool completely before driving on them again.

Do not engage the parking brake until after this cooling process is compete.

 

When you bled the fronts did you get good fluid flow out of the bleeders?

[hawaii_620]

Did you ever verify that the front circuit of the m/c is connected to the front lines?

 

Do you still have the stock m/c? If so, and it was working.....put that one back in and try that. That would "test" that part without having to buy another.

 

 

I checked the F / B lines and I have them correct. My old M/C was totally broken and leaking all over, so I tossed it. So I'll have to buy another one. If the M/Cs aren't any different between the Zs and the ZXs, I'm going to get the same one. If I find out the old one was broken, I'll put it in the new box and take it back to return. My truck has less than 100 miles on it since the conversion, so maybe the old M/C wasn't good when i bought it. It's a Checkers part, it's not unheard of to happen.

 

 

Can the wrong length rod coming off the brake pedal cause problems, to long or short? Same with the rod between the booster and M/C, this is a conversion, correct?

 

 

I adjusted both rods, the pedal to booster, and the booster to M/C. I'm pretty sure I have the pedal to booster rod good. Just a little bit of play before it engages the booster. As far as the booster to M/C, I did adjust it. But I'm a little tentative about it because I heard that if I make it too long, I could break the M/C. I guess I could adjust it a bit more. But how would I know when i've hit the optimal length without going to far?

 

Yes, it's a disc brake conversion for the fronts. I left the back as drums.

 

 

When you bled the fronts did you get good fluid flow out of the bleeders?

 

I'm going to try and break in the pads accordingly. I haven't driven the truck that much because of the lack of brake power.

The flow from the front calipers when bled looked normal/good though.

[thisismatt]

Such horrible braking performance wouldn't be from poorly bedded brakes alone, so I wouldn't bother trying to get them bedded correctly when they're not working even close to where they should.

[mklotz70]

True, but glazing over a new set of pads doesn't help. It doesn't sound like he's been able to brake hard enough to cause it anyway.

[mklotz70]

did this ever get fixed?

[jesusno2]

does the 75 have the valve under the bed? those are such a whore to bleed. Only way i can get the air out of one of those is with a vacuum bleeder i actually have a pump i made for a kc i worked on.

[hawaii_620]

I still haven't fixed this issue with my brakes yet.

 

I did however get a junk yard MC from a early 90s hard body, and I'm going to try and see how that works out. It's a 1 inch bore, so I'm worried that it will make my pedal hard to push. Hard or not, if I can stop, it will be a lot better than what I have now.

 

I'm also considering buy the SSBC Brake Pressure Gauge Kit so i can actually find out what type of psi I'm getting to each wheel of the truck.

[hawaii_620]

Still waiting for my NEW master cylinder from NAPA... 4-6 weeks special order... the price I pay to be in Hawaii.

[DailyDato]

When you did your conversion, What calipers did you use from what vehicle and what bore size, what is the bore of your wheel cyclinders, and what is the bore size of your master cylinder, its a simple problem of Fluid dynamics, I just learned about all this when I got my A.S.E in Brakes at school, its a simple miscalulation in your bore sizing and not a flow problem, the master cylinder will move the same amount of fluid front and rear, its the effect of the displacement inside your calipers and wheel cylinders that makes the brakes work, If you can provide me your bore sizes, I can help explain the neccessary fix with out spend loads of money and waiting weeks for parts

[hawaii_620]

I used the D21 calipers (88-97 4wd, 2.4L) , my back wheel cylinders are new (stock size), the master cylinder is a 15/16 from a 240 (don't know for sure if it's a ZX or a Z). After getting these parts from "my parts guy" at a local parts supplier, I've heard that he makes a lot of mistakes in getting the correct parts to people. So although everything bolted up as specified, I can't 100% say i know for sure if the parts I asked for are for sure the parts I got.

 

I just got my new (not reman) brake master, so I'm going to try it this weekend. I hope it solves the issues.

 

I'm guessing the master cylinder seals are bad. It's firm with no booster connected, and then with the booster connected it sinks to the floor without any braking power. Maybe just the front chamber of the master is bad.

 

I'll find out soon enough.

[DailyDato]

The 4wd D21 Calipers, Are they dual piston? if so, I believe the issue would be that your rear wheel cylinders are too small, but if your master holds pressure with no booster vacuum, n sinks to the floor with vaccuum applied, then you still have air in your lines, mainly your fronts,especially if you get good reverse rear brake responce, but if your calipers are dual piston, your rear cylinders are too small, upgrading the master to a larger bore, doesnt have the actual effects on your brakes as to how many think, i always thought that larger bore, more fluid capacity, better braking, but actually it just creates a firmer pedal and requires more effort to brake.....it all has to do with fluid volume, and surface area, surface area of the master versus combined surface area of your rear axle, versus the front, by means of your wheel cylinders, and calipers.....if your master swap doesnt work, lemme know, ill get out my books and run specs of your parts, n figure out whats goin on, and see if i can offer help

[hawaii_620]

Tired to bolt up my new master today... new lesson learned at the expense of $80 and 6 weeks of waiting... a 78 280Z master does NOT fit, or at least not all of them do. Apparently they have different lengths on the inner side of the mounting flange (side that touches the booster bracket). 1/8 inch difference. Just like starters I suppose... correct year, correct car, but then you find out there are three different models for one car.

 

I can't remember what I even asked the person at the counter when I bought the part, it's been a while. But my receipt says "280Z". I guess I should have asked for a "280ZX"

 

The parts list for my front disc conversion brackets said 280zx and 280z, so please note to anyone else that is doing a conversion... check your parts for proper fitting. (I should have done this before I bench bled the MC, because now I'm pretty sure I can't return it)

 

Anyone interested in buying a Z car MC?

[mklotz70]

I thought there was a write up or, at least, more info on the m/c mounting. It was fairly common knowledge a couple of years ago, but I can't seem to find a write up on here.

 

You need to open the firewall hole up about an 1/8" and elongate the mounting holes on the m/c with a file. You will also need to fab a new push rod since the style changes.