Rear Brake lock up (1 Viewer)

[mikesc]

I have done a RDB conversion with Monte Carlo calipers in the rear. The front are stock 40 calipers. All lines and calipers are new. I bought a new master from an 80 series from JT Outfitters. It has the front port come out of the top. The rear has alway locked really easily. I just installed a wilwood prop valve. It really did not make much of a difference. I did a search and did not find a fix. Has anyone had this problem and solved it?

 

[Degnol]

Have you got the proportioning valve screwed all the way counter clockwise? That gives you the most restriction to the rear. The reason I ask is...............I did just the opposite as it seemed right


Ed

 

[35inchoverdrive]

We tried both ways(I was there helping *not* fix the brakes).

 

[spotcruiser]

And you have the Wilwood valve installed correctly (inlet and outlet)?

 

[Degnol]

May be a stupid question, because I know nothing about 80 masters, but did you remove the residual valve from the rear circuit?

Ed

 

[mikesc]

The wilwood was put in in the correct inlet and outlet. I dont know if there was a residual valve in the rear line. It was a master for 4 wheel disks from the start. I did not think that I needed to remove anything.

 

[mikesc]

Anyone else got any ideas? I dont know what to try next.

 

[Fast Eddy]

May be a stupid question, because I know nothing about 80 masters, but did you remove the residual valve from the rear circuit?

Later model '80 series '93, or '94+ came with RDB, so it wouldn't be an issue if they got the right master, and the residual valve would cause them to drag, not necessarily lock up easy.

 

[Fast Eddy]

Anyone else got any ideas? I dont know what to try next.

This is the first tech article mentioning a specific valve that comes up in a google search, and it's a Wildwood. I was going to suggest trying a different brand, but it seems you made a popular choice.

Install a Brake Proportioning Valve - Tech Article - Chevy High Performance Magazine

They show a pressure gauge. Sounds like a good next move.

I've had enough new brake parts that were bad out of the box to not be too surprised if that was the case.

Another one not far down mentions Wildwood as well.

Installing a Wilwood Proportioning Valve - ThirdGen.org

 

[Downey]

I guess I'll go ahead and show my ignorance on FJ80's now. I thought the FJ80's never had rear disc brakes, only the 100 series---guess I was wrong. The FJ80 master cylinders are all the smaller 1" bore, and I would have assumed they did have the built-in 6 lb. residual valve that had to be removed
(a) if your proportioning valve is in fact working, you shouldn't have this problem.
(b) had your Monte Carlo calipers been ratchetted-up too tight before installation, you should not have been able to get them over the rotors???
(c) I would have a professional brake shop actually measure front to rear pressures, but don't be surprised if the numbers are not as high as you expect. The primary issue is that the numbers show roughly 60% front pressure to 40% rear pressure.

 

[dgangle]

Mike,
I got your PM. I figured I respond here for the benefit of the collective knowledge base.

I too had/have the same hardware/problems you have described. While many of the suggestions are well-intended, they go to the lowest common denominator and can be taken, well, kinda personally if you know what I mean.
I don't think I'm that stupid.

Others have stated they have the same exact set up but don't have this problem. I figured I am an anomaly or they don't know what they're talking about. You chose, could be a little of both!

I tried one adj prop valve and the bias got better but still not quite enough. I put in two. It wouldn't lock up the rears anymore but it wouldn't stop either.

I installed the Coleman piston reducers (another hundred and a half down the drain including custom machining to make the fit properly) figuring this a problem the circle-track folk have solved. They use the same exact GM metric caliper. It wouldn't stop again. I took them out.

It isn't a long-term solution but I have just learned to live with it. The front/rear bias problems I have show up worse when the hardtop is off (weight), it's wet and/or I am going down hill. Add a couple people, the hartop and a small utility trailer and it stops wonderfully. I think I only drove my 40 something like 350 miles last year all on nice warm topless days on road shuttling my kids around. Yes, I know I suck.

I have bought/used/measured and documented the front/rear pressures using a SSBC kit. All were in line with what they should be. I don't recall what they were..I have it written down at home in my notes.

In the end the problem is still there and I live with it. When I drive, I drive it like I prolly will when I'm 50 instead of the 17 year-old in my mind. I go slow in congested areas and leave lots of room to the car in front of me in the event of a panic stop.

Hey, anybody got a solution other than 'mine doesn't do that' or something obviously foolish, I'm all ears.

https://forum.ih8mud.com/40-55-series-tech/63132-brake-caliper-reducer-rdb.html

 

[e rock]

It really just sounds like your front brakes aren't all they ought to be. I don't know/didn't see what year 40 you're working on, but have you tried replacing the front calipers/pads with either new or upgraded/bigger ones?

I don't want to tell you to throw money at something that 'might' be the problem, but if the line pressures are right and you've tried everything else, the fronts seem like the next logical culprit.

 

[dgangle]

Good point erock.

Not to hijack, but I have measured the brake fluid pressure at the MC outlet to the front circuit and at both front calipers. They are the same. I have made sure the pistons on my new largest piston available $150/each V6 4R calipers are not seized/frozen/restricted in any way and have tried 4 different pad manufacturers/compounds all to no avail. At least I have a lifetime supply of front pads! Anybody want to buy some, I'll sure never wear out 3 extra sets.

I've all but given up and my engineer cause/effect mindset kills me.

Good point though. Maybe I should investigate this further but I picked all the leaves from my money tree and it died.

 

[e rock]

Good point erock.

lol. It sounds like you've been down the exact road I was pointing to.

What's that saying about free advice? :whoops:

 

[dgangle]

lol. It sounds like you've been down the exact road I was pointing to.

What's that saying about free advice? :whoops:

All well intended! Doesn't hurt to look at a problem through another's eyes.

 

[Fast Eddy]

How about reducing the pad area on the rear brakes? I don't know how you'd do it without breathing a bunch of really bad stuff...

 

[KOOTENAY KRUZER]

My '76 FJ40 and my '79 BJ40 will lock up the rears before the fronts when lightly loaded with the original DRUM brakes if they are adjusted properly. They were both brand new from the dealer when I bought them. I always thought it was because there was no automatically adjustable proportioning valve for the rear brakes so the Toyota engineers picked a compromise bias that worked better with the vehicle loaded (remember, these are utility vehicles, not sport utility vehicles). In a panic stop everything locks up if the rears are working correctly. I find with my '83 BJ42 which the PO put Monte Carlo discs on the back the rears lock up easier than I would like - there doesn't seem to be enough pressure on the fronts when the rears lock up, but then I can't find a proportioning valve anywhere on the vehicle and I have looked.

 

[bsevans]

I guess I'll go ahead and show my ignorance on FJ80's now. I thought the FJ80's never had rear disc brakes, only the 100 series---guess I was wrong. The FJ80 master cylinders are all the smaller 1" bore, and I would have assumed they did have the built-in 6 lb. residual valve that had to be removed

FJ80 '91 to '94 have 4 wheel disk and can be used in a FJ40 rear disk conversion right out of the box. That would be a direct bolt-in. The only alteration would be to one or both of the brake lines to line up with the new MC ports. Both the MC for non ABS and the MC for ABS systems can be used. I did not use a proportioning valve. I have the Wilwood but have not seen the need - yet. I would be surprised if the Wilwood PV is faulty from the factory. They are a first rate company.

Wilwood

This is a '91-'92 FJ80 MC mounted in my '78 with a Monte Carlo rear disk conversion.

 

[e rock]

FJ80 '91 to '94 have 4 wheel disk and can be used in a FJ40 rear disk conversion right out of the box. That would be a direct bolt-in. The only alteration would be to one or both of the brake lines to line up with the new MC ports. Both the MC for non ABS and the MC for ABS systems can be used. I did not use a proportioning valve. I have the Wilwood but have not seen the need - yet. I would be surprised if the Wilwood PV is faulty from the factory. They are a first rate company.

Wilwood

This is a '91-'92 FJ80 MC mounted in my '78 with a Monte Carlo rear disk conversion.

???

91-92 had drums in the rear. But perhaps that makes the brake proportioning closer to correct for GM calipers on the rear?

 

[John Smith]

I wonder if tire size is a factor in the rears locking up? I have the gm discs on the rear and a wilwood valve reducing the rear circuit as much as possible and I can't lock up my rears. I have unfortunately had to panic stop so I know this is the case. I am running 36" Irok bias plys on steel wheels.