Braking power question

[Tranz Zam]

Hey guys,

I have a question about the two letter codes on the master cylinder and the brake booster. They're different codes, so they don't match up with each other. The truck stops, but it doesn't seem to bite very hard, and wont lock up the wheels.

The master is out of an 87 C10. I have no idea what the booster is out of. The original 87 C10 booster lost its pushrod, so I picked up a used booster with a pin. Initially I put the pin from the new booster into the 87 booster, but it looked like it stuck out too much, and hit the piston in the master cylinder before it was bolted flush to the booster, so I just swapped entire boosters.


The entire brake system on my truck is new, with the exception of the master, booster, and proportioning valve. Let me break down what I have on the truck:

New front calipers
New rear wheel cylinders
New pre bent steel lines
New rubber hoses for front and rear

I've bled the system 2 times now, and there's no air in the line that I can see.

I don't know if I'm just losing my mind here, or something is actually not matching up.

Any help is greatly appreciated guys, this is the last thing I need to figure out before I can drive the truck!

 

[Tranz Zam]

I just got the codes off the booster and master:

Master = ZF

Booster = FD

 

[Tranz Zam]

I found the old VIN for the cab, and it comes back as a 1987 2wd Chevy 305 TBI with a brake system GVW 5001-6000.

I'm assuming that this master is completely wrong for what I'm running. The frame on my truck is a 1978, with axles and brakes to match.

I'm thinking I should get a master cylinder for a 78 K10, but what about the booster? Will the FD coded booster work with a 78 master?

 

[shiftpro]

Sorry I can't answer your particular questions but have some 2 cents anyway.
I understand boosters can wear out but still work, just weak.
Your master cylinder may need bench bleeding if you haven't done that.
I have heard of proportion valves getting plugged with crap, and this would effect something one would think.
If you hadn't already replaced about everything I would say go juice brakes. I have done this mod to a few of my trucks and THAT is the hot setup imo. Juice brakes from a 1 ton... That will put flat spots on your tires, even big ones. Good luck getting your stoppies down.

 

[chengny]

Did the brakes work properly before you made all the renewals?

How about shortening the booster output rod - the one that didn't fit.

It would be almost impossible to try and diagnose this due to all the variables you have going on.

It sounds as if the master cylinder is leaking internally - or there is insufficient stroke being applied to it.

You have a 1987 MC - coupled to a boost unit of unknown vintage and application - and they are being actuated by brake pedal assembly that was designed for use with a 1973 MC/booster setup. That is what is making me think the MC isn't getting a full input stroke.

The boost unit has nothing to do with it. As long as the boost output rod is extending out far enough to fully stroke the master's piston (and the master is good), when the brakes are applied you should be able to develop full pressure in the hydraulic circuits. Brake assist systems are designed to do just that - assist. Even if the brake assist system is completely non-functional, a healthy adult male should be able to cause a brake lock-up condition without too much effort.

 

[Tranz Zam]

Yeah, there's a million things going on with this truck lol.

The brake pedal assembly is also from an 87. So I would think it should play nice with the 87 master.

I was flipping through part numbers and various other info, and I believe in 87 a 2wd and a 4wd half ton shared the same master. I would have guessed that the piston diameter would have been different.

I'll bench bleed the master. I cant remember if I did it now, so I'll do it again to eliminate that potential issue.

 

[da_raabi]

What you may be running into is that the booster does not match the pedal. These two parts must be matched together. What you are experiencing sounds exactly like what happens when you use a 1979 Trans-Am dual-diaphram booster (designed for 4w disc) with a standard brake pedal designed for the single booster. The pedal ratio is wrong for the booster, resulting in a short-stroke of the M/C - meaning weak brakes.

I used the T/A booster in my Camaro and had to re-locate the pin on the brake pedal to ensure the correct ratio. It sounds like this is your issue. The booster of unknown origin is mismatched to everything else.

Since it is apparent you are not afraid of spending money on your truck, match your parts up. Pick a model year and system, and get all your parts matched up. So fill this out:

M/C Make and model year:
Booster make and model year:
Pedal make and model year:
Brake "system" (discs/drums/prop valve etc) make and model year:

 

[Keith Seymore]

I just got the codes off the booster and master:

Master = ZF

Booster = FD

They won't match, because they are individual parts (not an assembly).

Those are the assembly plant broadcast codes for the specific part numbers.

ZF= 18008065

FD = 18010581

It's easier for the assembler to remember to grab a "ZF" than it is an "18008065".

K

 

[Keith Seymore]

The brake pedal assembly is also from an 87. So I would think it should play nice with the 87 master.

This is correct.

The design intent pedal ratio for the ZF/BD combo is 4.49:1.

You can check this on your truck by measuring from the brake pedal pad to the pivot point, and dividing that by the distance from the brake pedal pin to the pivot point.

K

 

[Keith Seymore]

I knew I had this but couldn't find it -

Here is the brake system release chart for 1987 R series trucks. These are the charts the design engineers would create and refer to when releasing the system for production and service.

K

You must be registered for see images attach

 

[MikeB]

A few comments:

As per Keith's document, your ZF master cylinder was used with the common 4.49:1 pedal ratio. The higher pedal ratios are used with manual brakes (6.50:1) or hydroboost (5.60:1) and would cause your system to have a "grabby pedal, due to too much leverage -- just the opposite of the problem you're having.

Looks like the M/Cs had either a 1" bore (25.4 mm) for manual brakes or 1-1/8" (28.6 mm) for non-hydro power brakes. So your ZF master should work with your FD booster and a 4.49 pedal ratio. Yes, there is also a CM booster/ZF master combo, but since there are no booster specs, I can't tell the difference between the CF and FD boosters. Could be a larger diameter diaphragm on the FD booster.

Anyway, assuming you have a 4.45 pedal ratio, you have the booster and master cylinder shown in the 4th column of Keith's document. Looks like JB5, but so is the one in the next column. ???

Also, have you checked the booster by sucking on the hose?

And I think the suggestion to bench bleed the master is a good one. I typically do it installed on the truck. It's less messy and just as effective, if not more so. All you need is a helper to pump the pedal. I'd bet $1 that you'll get some air bubbles!

Finally, don't forget the rear brakes. Make sure the full arc of the shoes contact the drums. New shoes on turned drums makes for contact only in on the center part of the shoes, thanks to the death of "brake shoe arcing" machines.

 

[MikeB]

Interesting that Keith's document shows two different combination valves. A lot different than the one-size-fits-all aftermarket valves. As if what works on an 3/4 ton truck with 30" tires would work on a 2700-lb Nova with 26" tires.

 

[Georgeb]

Also note the rear wheel cylinder diameter. I went to the larger ones on my 78 and while it changed the brake feel and stopping power it also made it easier to lock up the rear brakes in an emergency situation.

 

[Tranz Zam]

Lots of great info guys! Thanks for the help!

I haven't had the chance to dig into it yet, but I should by the end of this week. So the master and booster should be playing nice with each other, as well as the brake pedal assembly itself. That's good to know!

The booster seems fine. Its not making that awful whoosh noise that they make when they start to leak.

Just to add to the confusion, I have no idea what year the proportioning valve is. I know its for a 4wd truck, but that's about it.

Speaking about wheel cylinders, I ordered replacement ones for a 78 (year of the frame and axles). IIRC they're 15/16" bore. I had one take a dump on me before I even got down the driveway. When I took it apart it looked like the little push pins extended too far out, and it gave up. I replaced it with another one. So far no leaks.

 

[MikeB]

Also note the rear wheel cylinder diameter. I went to the larger ones on my 78 and while it changed the brake feel and stopping power it also made it easier to lock up the rear brakes in an emergency situation.

Good info there.

The same PSI from the master cylinder applied to larger pistons = greater force on the shoes. Too big and the rears lock-up too easily especially in an unloaded truck.