Brake question

Suspension, including wheel, tire and brake.
vetteguy22
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Brake question

Post by vetteguy22 » 24 Jul 2018 22:26

I’ve installed 280zx front brakes with drilled and slotted rotors and maxima rear calipers with drilled and slotted rotors along with new braided ss flex lines on my 1971 4 dour 510. AlsoI installed a new 13/16” master cylinder and have bled the hole system. I’ve got brakes with no air in the system since the pedal is firm and steady but am not happy with the braking. I would think that I should be able to lock up the brakes if I mashed on the pedal. It slows the vehicle to a stop but am worried that in an emergency situation I wouldn’t be able to stop quickly enough. Any ideas?
Thanks,
Rob

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Byron510
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Re: Brake question

Post by Byron510 » 25 Jul 2018 07:23

Just to clarify, is it the front or rear brakes that you are having issues with?

In any case, yes the brakes should lock up with you standing on the pedal.
There are a few possibilities for problems, I'll just toss a couple out there. It is possible that the brake pads are not yet bedded in - making proper contact with the rotors if everything is new.
And do confirm that all the packing oil was cleaned off the rotors before you drove the car for the first time. Otherwise the pads will be contaminated and not work well at all. It's an easy mistake to make, I've seen it done before.

You may find that the 13/16" BMC is a bit on the small side, in that it will let the pedal travel further to achieve the same amount of braking force applied at the calipers (F&R), but will require less effort so it makes the car easy to operate. If the pedal travel does seem excessive, I'd suggest stepping up to the 7/8" BMC out of 240/260/280Z (S30 Chassis) M/C. It's a straight bolt in, both M/C's are identical save for the bore size. Some 260/280Z M/C's have the bleeders pointed at the inner fender which sometime helps with clearance issues depending on your intake set up.

The next step up is the 15/16" M/C from the 79-81 280ZX. This is the actual size of M/C that the brake system in the 280ZX and Maxima utilized, however the pedal ratio was different so it all comes out in the wash. However if you want a really low movement pedal travel with more force required, you can also go this direction for a straight bolt in. Note that the 82/83 280ZX has a horizontal bolt pattern, and will not bolt in. Last note, Z31 300ZX has a 1" BMC bore size, but that's getting way out there and again a different BMC mount pattern.

If the problem is just the rear brakes, there is more, but I'll let you confirm first.

Hope that helps a little.

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Re: Brake question

Post by bertvorgon » 25 Jul 2018 07:24

Silly question maybe...what is the pad compound? You said NEW rotors, is everything broken in together yet?

Sometimes, getting pad material transferred to the rotor is when they really start to work.

I finally found that running too hard a pad compound on the street was just a waste of time on our light 510's if not actually racing. I run as soft a pad as possible.

I did go to the 7/8" master also.
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vetteguy22
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Re: Brake question

Post by vetteguy22 » 25 Jul 2018 14:38

The rotors were cleaned upon install. In fact the other day before driving it to town I had noticed the rotors had a slight bit of rust due to being outside during the last rain.
I am unable to lock up any of the brakes but believe the fronts are doing the majority of the work.
The brake travel is acceptable and this will be my wife’s car so easier pedal is good. Just want to make sure it’s safe.
It’s possible that they haven’t bedded in yet I guess.
I think I’m going to put the car up on the lift and check to see if I can tell if the front or rear are having more of an issue.
Thanks,
Rob

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Re: Brake question

Post by Byron510 » 25 Jul 2018 16:42

The combo should be fairly well balanced, as far as braking goes. But you should be able to lock them up no problem - even on the stickiest R compound tires on a dry road. If you can't then something isn't right for sure.

As Bert mentioned, check to see that the front pads are actually biting into the rotors - is the pad material too hard? Did someone box the wrong pads in your regular box...

The same goes on the rear, but there may be one more possibility. There is a brass block, right behind the cylinder head that simply looks like a connector for the front line from the pressure differential switch to the rear line that comes up the tunnel. Well, that brass block has one more job - it contains a restrictor hole that limits the instantaneous amount of pressure to the rear brakes by utilizing a small orifice - essentially restricting flow. Now not much fluid has to move through to activate the brakes in the first place - this is noted, especially on disc brakes. But it could be a problem. Ask yourself how the fluid movement was when you were bleeding the brakes? This should give you a reasonable answer.

But this still doesn't explain why your front brakes don't work - they should lock up no problem, so I'd start there. Stand on the pedal as had as you can. If it hits the floor before the brakes lock up, you need a bigger BMC. But if you can't bottom out the pedal, the hydraulics are good and you'll need to look at the caliper condition (new I presume?), rotor and pads next.

-stand on brake pedal as hard as you can, it should never bottom out under any circumstance.
-bed pads front and rear to get proper brake in and best levels of pad braking efficiency
-check fluid flow to the rear calipers

A few thoughts anyways.

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Re: Brake question

Post by vetteguy22 » 25 Jul 2018 16:54

Ok put the car on the lift, started it and placed it in gear, automatic trans. I’m able to make the rear wheels spin easier than I believe they should with giving it gas. At idle it holds them still.
Had my wife apply the brakes and I attempted to rotate the front tires. No way, very tight. Then the same with the rear tires. I’m am able with great effort, to rotate the rears.
Hope this gives you guys some ideas on where I should look next.
Thank you for your help,
Rob

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Re: Brake question

Post by vetteguy22 » 25 Jul 2018 17:30

Byron,
The only real new parts in the front are the rotors and ss flex lines. The car had the 280zx set up when I purchased it and stock drum rear.
The fronts seemed to work fine but never really drove it much due to other issues. The front rotors were bad so I replaced them and rebuilt the calipers. I used the original pads since they still have plenty of thickness. I did spray them with brake clean and scuff them with 80 grit sandpaper before reinstalling them.
The rear wheel cylinders were leaking so finally installed the rear disc set up.
New calipers, rotors, pads and ss flex lines.
The stock 3/4” master cylinder was leaking so I installed the new 13/16” master cylinder.
I did have a hard time bleeding the rears. I picked up a vacuum bleeder and was able to get the air out and the pedal does not go to the floor and is firm when applied. Not sure but seems like the pedal comes up a the slightest bit when hit a second time but doesn’t affect performance.
Hope this helps,
Thanks,
Rob

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Re: Brake question

Post by Three B's Racing » 26 Jul 2018 05:28

Is the master cylinder setup for rear disk brakes or drums? Because that will make a big difference.
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Re: Brake question

Post by vetteguy22 » 26 Jul 2018 06:08

How can I tell if the MC is set up for rear drum or disc?

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Re: Brake question

Post by Byron510 » 26 Jul 2018 06:38

Three B's Racing wrote:
26 Jul 2018 05:28
Is the master cylinder setup for rear disk brakes or drums? Because that will make a big difference.
Eh Lou, good of you to chime in.
Curious, what design change is there that makes a BMC a disc or drum BMC? To me a BMC is simply a piston pushing fluid through a bore. The only design feature in addition to this that I’m aware of is residual pressure valves often found on drum brake systems that are designed to keep the pistons expanded keeping the shoes closer to the drums. But I don’t believe any of the Datsuns used this feature that was more common on North American drum brake systems.

Just currious, that’s all. Thanks.

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Re: Brake question

Post by bertvorgon » 26 Jul 2018 07:51

Just an FYI:

I never changed any of the stock brake lines, other than having stainless for the hard line to caliper. I have a Direct Connection Bias adjuster, which works, 7/8 master. I even left the stock brake warning switch in.

I use the BMW 2002 FRONT caliper on the rear, 4 piston, and the Brake Man Mini-GN size four piston caliper up front, mounted to my 280zx struts with rotors. Never had an issue with braking, nor being able to send more pressure to the rear, and that is with having left that small restriction block in place.

My biggest issue has always been getting a pad up to temp and proper transfer of pad material to a rotor to establish proper GRIP.
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Re: Brake question

Post by Byron510 » 26 Jul 2018 09:06

bertvorgon wrote:
26 Jul 2018 07:51
Just an FYI:
My biggest issue has always been getting a pad up to temp and proper transfer of pad material to a rotor to establish proper GRIP.
Good morning Keith - funny that you and I were talking pad bedding this past weekend briefly, as I experiences green pad fade while braking in my new Polymatrix pads in the days before our drive - obviously I got them too hot during the brake in procedure for green pads.

Do you have trouble on the fronts or rears getting them up to temp?

I was careful that I did bring the new front pads up to temp slowly as suggested by Wilwood;

https://www.wilwood.com/Pdf/Flyers/fl384.pdf

Using some hardly used and dead end roads out on the flats by Pitt Lake, it was 10 minutes into the procedure before I hit the green pad fade mark. Of course I let them cool and then continued with the bedding procedure and all went well. The Polymatrix compound is an impressive product, but they have their drawbacks at the expense of rotors and metallic dust that you need to be aware of.

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Re: Brake question

Post by Byron510 » 26 Jul 2018 09:46

I also have my doubts that the stock real line restrictor is the issue in this case, something else is not right.

The brake pedal is now confirmed firm under all circumstances, so I rule out any BMC sizing issues - really that's just preference of pedal travel vs effort - and for those of us that track a car - pedal travel is a real issue when things get really warm! Street car - it's not usually an issue at all.

Something else is not right - either there isn't enough hydraulic pressure reaching the calipers, the calipers themselves are not working or the pad/rotor combination isn't correct in some way.

The bottom line is that you should be able to stand on the pedal on dry roads and lock up the brakes no matter what tire combo you are using, and this test should be performed anyways on a new brake system - otherwise you can't prove that your bias (F/R) is correct.

So coming back to you front brakes; The rotors are new, verified clean and hand scuffed before installation. Calipers are across the counter rebuilt units, but pads are original and from unknown origin.
You also state that the front rotors were bad - were they worn badly? If this is the case, the old pads would have worn to the taper and shape of the old rotors. Did the old pads get overheated and become glazed or checked? In this case the pads may not be making contact with the full rotor surface, in fact they may be point loaded and won't work very well at all. This should be clearly visible when you inspect them as contact marks should be visible after the first drive. Resurfacing the pads completely flat and parallel to the backing plate is a must for proper pad/rotor bedding. Or simply replacing the pads with a known new, clean, uncontaminated pad of known type and quality may be a very good step here. Who's to say someone didn't previously buy a really hard pad thinking that this was a good hi-performance mod to do? Or the pads were previously contaminated and there's no amount of surfacing that can fix this as the material is porous (ask me how I know this?). You would eliminate the guess work up front.

On the rear brakes, there are other issues. Up on the hoist, there's no way you should be able to apply enough torque by hand - or even with a cheater bar, to overcome the brakes when fully applied. So there is certainly something not right here.
You have verified that you get good hydraulic pressure to the rear - when you bleed them you get good fluid movement?
You mentioned all new parts out back which is good.
Can you confirm that the packing oil was first cleaned off these rotors as well before the new pad ever touched them in any way? Just checking :-)
Do you know exactly what the pad material is? Again is it a soft street pad or a harder low wear compound?
Assuming hydraulic pressure is good, what kind of pad to rotor contact can be verified - is any wear yet present on the new rotors?

Lots of time spent so far, but we're trying to help.

Regarding bleeding of a new BMC - yes sometimes they are a real PITA to get fluid moving - especially to the rear brakes. Once you get fluid moving through the system, it all goes well. I remember once fighting for hours to a get a new BMC to start moving fluid - it just wouldn't start. I think I ended up forcing it through with compressed air before it finally got going, but then it was no problem - I remember this being quite frustrating when all you needed to do was bleed the brakes. There is certainly something to be said for prior bench bleeding before installing the BMC in the car. I know we don't want to drop brake fluid anywhere, but it sure saves time and frustration if you do complete this step on the bench first.

Anyways, lots to check - hope to hear back from you with your diagnosis.

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Re: Brake question

Post by bertvorgon » 26 Jul 2018 09:56

Your question is kinda hard to answer. As most of you know on our drives, I hardly EVER use my brakes going into a corner. For that matter, our drives have just constant speed, so heavy braking for the most part is never used.

I put new pads in last spring, with new front rotors. The pad compound all around was recommended by Brakeman for the type of driving I described and basically said, "I WANT BRAKES WHEN COLD!"

When Norm and I blasted back though the American side of Sumas Prairie, we accelerated hard from corner to corner and that was the hardest use of my brakes in a long time, which only had them just getting up to temp, don't forget I have those proper brake cooling ducts on there too.

So, I followed the brake in procedure for the most part but never got them to the "green fade" point. At this point I have many drives on them, but, I can still see cross hatching on the front rotors. This past trip I only used them hard once when sucking up to the bumper of Trevor's Porsche, when he did not carry speed through a corner, I had no problem with slowing. The pads were right there and judging by the "feel", I can tell they will be awesome when actually up to temp. Brakeman's recommendation looks to have a wide latitude in temperature, short of all out track racing, more than enough for my use.

That all being said, for the zillion years I have had my car, getting the brakes to put some heat in our light cars with the larger 280ZX rotors has to be an issue when from dead cold, that has been my experience, but then I always ran way too hard a compound on the street. These new pads SEEM to have finally solved that problem and are getting better each time I actually use the brakes.

When I last ran Knox in 1997, I put as soft a pad as I could in at the time, to try to get some grip. The years I ran "race" pads...useless really.

Yes, I am familiar with the Polymatrix issue and did not want that, which is why I asked Brakeman for their recommendation. That killer dust I did not want anything to do with and so far, these pads of mine have minimal dusting...but then....I hardly use the brakes.....

Not sure what Rob's problem could be, you would think if stock pads, that once bedded, should have grip as designed by the factory...IF the pads are a direct type replacement for factory and NOT a harder compound.

I stuck with FACTORY for my G35 and am glad I did, as they work best as the car was designed for, very quick and lineal grip when stone cold.
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Re: Brake question

Post by RMS » 26 Jul 2018 12:07

I can lock up a 31in tall tire with stock brakes, must be something funny going on....

with one rear caliper bleeder open, push on the pedal. is there any more resistance than the return spring on the pedal ? if yes the problem could be in a union or one of those new ss lines. just because its new doesn't mean it good.
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