Strange '78 620 4X4 brake problem...

[620Turbo4X4]

Hello,

 

I just picked up a '78 620 4X4 with a odd brake pedal problem...

 

If i'm stopping normally or slowly the brake seems perfect and pedal is very easy to push (could use a finger), but... when I really need to stop, the pedal continues to the floor then stops and gets rock hard halfway. At that point I'm pushing as hard as possible and clinging on for dear life (thinking I'm going to rip the booster off the firewall), and the brakes are barely stopping me. I have replaced everything with the exception of the metal lines, and rear brake hose. Once all the air is bled out the pedal is the exact same every time.

 

Truck is 4 wheel drive sitting on a dana 30 in the front, but still uses Datsun calipers and rotors mounted with custom brackets and a 6 bolt hub that I believe was made by Low Manufacturing back in the day. Calipers, pads, rotors, wheel bearings are all new and tight. I even took the hubs in and had them trued up by a machine shop and re-machined so the caliper pistons extended evenly on each side when touching the rotors. I'm on my 3rd. Master cylinder (This one was new), replaced brake booster (both made no difference), Removed proportioning valve (helped slightly as now I can lock the rears up if I push hard enough) and everything was replaced in the rear, shoes are tight.

 

Last night I made a plug and tried plugging the front and rear lines coming from the master individually, and the problem seems to be in the front. With the rear brakes plugged, no difference. Fronts plugged, and pedal is rock hard right at the top which leads me to believe the problem is in the front brakes. I had my wife step on the pedal and don't see anything moving excessively on the calipers. Looks normal and they seem tight and drag somewhat on the rotors, and, I know if they still had air in them, (i think I've flushed 3 gallons of brake fluid through it the past year!!) the pedal would be mushy. 

 

Anyone have any ideas?

[]2eDeYe]

Can you take a picture of your front brakes for me? 

[620Turbo4X4]

Sure if I can figure out how. New to this. Just joined the forum today.

 

The fronts are just the typical 78-79 620 disk brakes. but mounted to the jeep front end with basically just steel plate that has threaded ears to mount the caliper, and fits on and around the dana 30 spindle/studs. The hub is a custom affair, and machined fit fit the 620 rotors and 6 lug wheels with the dana 30 bearings and such on the inside.

 

I'll work on that photo. I need to upload it somewhere?

[]2eDeYe]

Sounds like an interesting setup. 

Sounds like your getting some cavitation somewhere or maybe your booster is going out.

 

Photobucket is the best bet, although it is a pain in the ass these days. How about some pics of the truck too :D

[620Turbo4X4]

I thought of that many times and have even replaced it… Didn't help. I'm pretty sure I got it narrowed down to the front calipers. If I plug the front port at the master cylinder. the pedal is right at the top. By plugging the rear, the brakes are the same. It's almost as if the front caliper pistons are getting "sucked" in too far and having to travel a excessive distance before contacting the rotors.. But then I don't understand why it is so damn hard to press the pedal at that point and stop? It get rock solid and has absolutely no give other than the firewall flexing.

 

Pumping the pedal stopped or moving changes nothing.

 

Strange huh?

[620Turbo4X4]

It's also strange that the normal braking is great, has excellent pedal feel and is super easy (I could literally use a finger). It's only when I need to stop quickly that the pedal continues down another inch then just stops….

[mklotz70]

My first thought is that you have a vacuum issue.  Under normal stopping, you have enough vacuum to keep up, but when you hammer on the pedal, it runs out of vac and therefore, no boost.

[datzenmike]

Check the one way valve mid way between the intake and the booster. It may not be opening all the way. Just remove it and try the brakes.

[yello620]

I don't think you have a booster problem at all.

Sounds way more like a brake pad quality problem. They work fine when used lightly, but under load they have no additional "grip", the fact that you have working power brakes allows you the assistance to push the pedal another inch or so. But the pads can't do any more work.

 

The other thing that I have seen cause this same feeling is brake pads that have not "bedded" in to the rotors yet. Also calipers sticking, the 78/79 design caliper has that crazy sliding bracket assembly that needs to work correctly for them to stop properly also. If that bracket assembly does not slide properly, then you basically have only one pad pressing on the rotor.

[620Turbo4X4]

I'm pretty sure is not a booster problem too.. The pedal reacts the same with or without vacuum (harder of course W/O vac) traveling half way to the floor and just stops, no give whatsoever. When I bleed the brakes front or back, it does go the rest of the way to the floorboard.

 

Calipers have been on about a year, so everything should be bedded in quite well. Sliding bracket has the teflon guides and works fine. They were loaded calipers so I'm sure they are cheap pads, but the truck had the same problem even with the old calipers that came on it.

 

I'm usually pretty good with this kinda stuff, but this ones really got me stumped...

[yello620]

Does this truck still have the NLSV valve on the passenger frame rail under the passenger door area?

[Laecaon]

I'm pretty sure I got it narrowed down to the front calipers. If I plug the front port at the master cylinder. the pedal is right at the top. By plugging the rear, the brakes are the same.

 

By front port on the MC, do you mean the port on the MC towards the front of the truck, or the port for the front circuit (which is actually the port on the MC towards the rear of the truck).

[620Turbo4X4]

Does this truck still have the NLSV valve on the passenger frame rail under the passenger door area?

 

 

It did, but not anymore. Have a wilwood proportioning valve that I'm going to install as soon as I can find the right fittings.

 

By front port on the MC, do you mean the port on the MC towards the front of the truck, or the port for the front circuit (which is actually the port on the MC towards the rear of the truck).

The front brake circuit.

[bilzbobaggins]

What size master cylinder?

[620Turbo4X4]

What size master cylinder?

Stock 13/16. All brake components are stock 620 Datsun parts.

[620Turbo4X4]

Can you take a picture of your front brakes for me? 

l

[620Turbo4X4]

Sounds like an interesting setup. 

 

Sounds like your getting some cavitation somewhere or maybe your booster is going out.

 

Photobucket is the best bet, although it is a pain in the ass these days. How about some pics of the truck too :D

…and some picts of the truck.

Still pretty rough looking.. Front end was pretty straight until a guy at work backed into it with a forklift.

 

 

Got most the brackets installed to install 4 link.. Still working on front and rear brackets for the 12" Fox coil-overs….

 

 

Fabbed up some new mounts so I can use the GM 105amp alternator.. Plenty of power now for fuel injection, stereo, and some good off road lights...

 

 

Installed '05 Grand cherokee Power steering box and 720 pickup pump. Original owner was using the stock 620 box and pitman arm connected to the jeep axle.

 

Pontiac fiero tilt steering column, Acura Seats with the floorboards reinforced so I could use the "seat mounted" seatbelt locks (don't need two hands to put the belt on :) )

 

I really hate the previous owners dash mods... I have new dash that I'm totally re-wiring with larger fusebox and relays for the injection. Haven't installed cause front window leaks...

 

 

 

Fuel injection Intake W/ 65 lb/hr. injectors on my mock-up truck. Need to find, or make a throttle cable for it now.

 

Still working on a stainless exhaust header to mount the T-3 Super 60 turbo under this….

[Guest Rick-rat]

Should't that be a flex line to the calipers instead of a hard line?. Probably gonna catch some flak for that fram filter too

[Cahoon520]

Should't that be a flex line to the calipers instead of a hard line?. Probably gonna catch some flak for that fram filter too

No that is how the 620 disk brakes are from the factory.

[Guest Rick-rat]

Just looked strange to me like that

[yello620]

Going back to what I was talking about before, of just running out of stopping ability due to pads that are not "bedded in" or just plain cheap pads.

 

Those marginal quality pads that are "bedded in" by now just might not be adequate to stop the extra weight of the 4x4 conversion and the significant increase in tires size that we can see now that you have some pictures up. Those 78/79 style front brake calipers on your truck have a very small brake pad to begin with. The pad just doesn't have very much surface area on the rotors. That minimal surface contact coupled with larger tires causes will make a drastic difference in how the vehicle stops.

 

If that was my truck, I would be looking into swapping GM knuckles onto it. Not sure if this is possible on the Dana 30 though. I would definentely be looking to get rid of those little brake pads and non-vented rotors.

[620Turbo4X4]

Going back to what I was talking about before, of just running out of stopping ability due to pads that are not "bedded in" or just plain cheap pads.

 

Those marginal quality pads that are "bedded in" by now just might not be adequate to stop the extra weight of the 4x4 conversion and the significant increase in tires size that we can see now that you have some pictures up. Those 78/79 style front brake calipers on your truck have a very small brake pad to begin with. The pad just doesn't have very much surface area on the rotors. That minimal surface contact coupled with larger tires causes will make a drastic difference in how the vehicle stops.

 

If that was my truck, I would be looking into swapping GM knuckles onto it. Not sure if this is possible on the Dana 30 though. I would definentely be looking to get rid of those little brake pads and non-vented rotors.

 

I do see your point about the cheap pads. I've had many of cars with inadequate brakes over the years and this doesn't react or stop like a pad problem. Plus it's not like I'm trying to stop from 70mph and the pads are smoking and "gassing" between the rotors. (Had that happen before and it's not too fun!!) It reacts the same sitting on or off jack stands my driveway, at 5mph, or 60 mph. Normal pedal for the first 2" of pedal travel, between 2"to 3" pedal continues down easily without providing any additional resistance, braking, or stopping power, then at 3" of travel it just stops as if it were bottomed out.

 

When I bought it it had 4 shot 235/75-15 tires (about 28" tall). I put these wheels on it about 8 months ago. (32-11.5) They are big, but really aren't that heavy. Tires have the kevlar sidewalls too. Brakes were the same with either set of wheels. I've put 1800 lbs in the bed of my other 620 longbed truck that has dump bed and dualys and it stops without this problem. I also had a Datsun 620 Mini motor home with a buick V6 transplant. I won't say either stopped well, but they didn't have this problem. 4X4 Truck only weighs 3080 with me in it.

 

If the rain lets up today I'll go swap the calipers off my other truck and see what happens.

 

I'm kinda thinking it might be something to do with the Dana front axle since I've never encountered this problem with any other 620 I've owned over the years, and no one here has had this problem. I just don't know what it could be. Wheels don't flop around, bearings new and tight and I don't see any excessive movement anywhere when the wife steps on the brake pedal.

 

 

What advantage would GM knuckles provide over the jeep knuckles?

[]2eDeYe]

What advantage would GM knuckles provide over the jeep knuckles?

 

You would get 1/2-3/4 ton brakes, vented rotors and much bigger calipers. Plus you get rid of the crappy 1st gen 620 disc issues.

 

To swap the knuckles you need to get dana 30 axle shafts that have dana 44 u-joints (I have a set I am not using)  Then you just need everything from the ball joints out from a wagoneer. This will keep you 6 lug. Another advantage is they are flat top knuckles and can be milled, drilled and tapped for high steer.

 

 

From your pictures it looks like you are probably running a Z'd draglink or you have it at a steep angle (this will cause a ton of bumpsteer). High steer helps flatten the draglink and allows it to be a straight shot.

[620Turbo4X4]

 

You would get 1/2-3/4 ton brakes, vented rotors and much bigger calipers. Plus you get rid of the crappy 1st gen 620 disc issues.

 

To swap the knuckles you need to get dana 30 axle shafts that have dana 44 u-joints (I have a set I am not using) Then you just need everything from the ball joints out from a wagoneer. This will keep you 6 lug. Another advantage is they are flat top knuckles and can be milled, drilled and tapped for high steer.

 

 

From your pictures it looks like you are probably running a Z'd draglink or you have it at a steep angle (this will cause a ton of bumpsteer). High steer helps flatten the draglink and allows it to be a straight shot.

For some reason I thought the 6 lug dana axles were for 1 ton trucks. I did some research on this on the internet over a year ago when I bought the truck... but pretty much forgot it all now (sucks getting old).I vaguely remember the 1 ton brakes were for 16" wheels and wouldn't fit inside my 15's or something like that?

 

My mock-up/parts 4X4 620 has the high steer kit. I was going to use it on this truck, but I don't think it will work out too well with the current setup I have, and the future 4 link and coil-overs (whenever I get that done). The Grand Cherokee steering box and pitman lowered the tie rod end down about 6" from where it was with the datsun box and the 12:1 ratio and longer pitman is sooo nice :). Bump steer IS a problem though. As you can see I've tack welded a much needed crossmember (factory one was cut out) with the attachment bracket for a pan hard bar to the frame. I'm not sure if the engine can still be removed with how close it is to the pan so I never completed the welds. When all said and done Panhard will be the same length and angle as the drag link and should get rid of most all bump steer. Front axle will be 1-1/4" farther forward when the 4 link bars are installed for more inter fender well to tire clearance. I'm trying to keep the truck low as possible so I don't tip it over, and still have 12" suspension travel. I figure It will sit about 2-1/2" higher than it is now, and I'll have the coil-overs at half travel in ether direction (50%) sag)

 

 

 

 

Never did get to swapping calipers… My machine shop finished turning down the shaft for my cam sensor for sequential injection. I got busy on that and chopped the shaft to length, cut the drive slot, and installed the Datsun sleeve... so that's all done now :)