do hydroboost brake systems have good brakes on older models like my 1989 p30

hi again everyone ,,im getting my motorhome ready for my winter fishing trips and i have had some issues and now im not sure if i have another issue or its just normal.i have a class A chevy allegro p-30 27ft long and it has the hydroboost system on it and in the 5 years ive had it the brakes are terrible and have always been no matter what i have changed ,it takes all i can to stop this thing sometimes and if its last minute ,its not stopping in time.i have always thought it was due to my rear brakes maybe being worn out and now i have replaced them and still no better brakes for stopping power than before.i have replaced over the years the master cylinder ,front brake pads ,rotors ,calibers ,brake hoses front and rear and now the rear brake shoes ,wheel cylinders and still it takes a block to stop this thing like the old cars without power brakes in other words ,thats just what it feels like.i have did the test to check the hydroboost to be sure it works and from how they say to test it, it checks good ,i think the test was to have it cut off ,then hold your foot on the pedal and start it and the pedal should come up if i remember right,its been a couple years ago since i checked it last.but i think that was how it goes..,but i have to think its either the hydrobooster or the power steering pump because theres nothing else left and the power steering works fine,no leaks or anything.so my big question is does any have a model year close to mine with hydroboost brakes and does it stop good or do you have to stand on it to slow it down????so i know theres nothing more i can do or is there another part i need to replace.the brake pedal is decent ,but if you have time to pump it a couple times they are a little better to stop.i have bleed them plenty all the way around and it dont really change anything as far as pedal,so thats not the problem,so any advice would be great as always im grateful for any advice on this issue ,i start my winter fishing trips the 4th of october and i want to be ready to go and feel a little safer going. i have another old 95 gmc van with hydroboost and it will throw you threw the window, but it is a little newer ,so i have no clue what to do, or not to do at this point..

  I ran into the similar problem years ago on f700 or f800 Ford. Turned out to be a restriction in brake line going to rear brakes. We found it finally, by putting pressure gauge between brake line and wheel cylinder one wheel at a time.  Time consuming,but it showed both rear brakes way down on pressure initially, but would build slowly. I think that?s right, it?s been 50+ years.
  I would think that today you might could take IR gun, do some hard braking, and maybe get pointed in the right direction.
  The gauge on each wheel routine isn?t really that bad if you have someone to work the brake pedal. You?ll need a 2000 lb gauge. I had to braze  up the fittings on gauge years ago, but I think you can buy everything you need today off shelf.

There is no doubt that the P30 requires a firm foot pressure to stop, but I have always found that mine will stop if pressure is put on the pedal.  Though it should be noted that P30 rear brakes are notorious of getting moisture in the brake fluid and do require brake fluid flushing every 1-2 years to keep rear brakes fully functional.

My 15 year total wreck had excellent brakes till 2 weeks ago (should have re-done the brakes a bit sooner 55,000 miles should have done e'm at aroudn 40 or 30 thousand)

It will have excellenty brakes when Next I drive it as well. .

Sadly the Total Wreck refers to tother damage but I need the RV to get to my new Sticks and Bricks wherever that winds up being.. Davison MI is looking better and better. I like the church. I like the music director. Will very likely end up in the choir there (Been ask to join) and even though  I don't care all that much for January in MI.. Well the State has many attractions.. Many attractions .

I don't think the brakes on any vintage P30 can qualify as "good" by modern vehicle stopping standards, but they were considered reasonable for their time.  Probably a bit skimpy for the weight of the vehicle, though.  A motorhome typically runs near its max (GVWR) weight, so the brakes have to work hard on every stop.

thanks for all the replies ,thats kinda what i guessed because it seems no matter what i change they really dont get much better at stopping.they are a little better than before because i used high dollar brake parts front and rear ,so i guess i will just have to keep good insurance and stand on that pedal.. thanks to everyone... 

You can also downshift to help out the braking...

Hydroboost brake pedal effort is directly related to the power steering pump pressure if I remember correctly.  The brake pedal does have a different feel than vacuum assist brakes but you shouldn't have to stand on it to get effective braking.

What happens if you press on the brake while turning the steering wheel all the way over to the end of travel so both the brakes and steering are loading the power steering pump?  If you feel pulsing in either the brake pedal or steering wheel you may not have enough output from the power steering pump.

i will check that out and see ,most of the time im going straight and the brakes are not great ,but i will try turning it and see how that goes and see what happens.it still has the original pump on it ,but it only has 67K miles ,but its 30 years old too ,so anything is possible at this point because you really have to stand on them if you have a fast stop.

Speaking of which have you flushed and filled your power steering fluid lately, when I changed mine after buying my coach at 75,000 miles it was black, it should be clear.

My 1996 stops like I threw out an anchor if I want it to although I am pretty sure I only have the vacuum boost system - haven't really looked.

For hydroboost he manual has a diagnostics check whereby a pressure gauge is hooked up to a bleeder port. You should get something like 1800+ PSI at the wheels. You may have to gather up some adapters and tooling but if I had done as much work as you I would be suspecting the hydro boost system.

I am not sure how it works but they also describe a two bleeding method.  The master cylinder is bled separately from the brake lines and there are two chambers in the master cylinder that require bleeding if air was introduced or suspected.

Do you have the old P30 chassis service manual? It has excellent tech info, including brake systems.

https://www.gmcmi.com/wp-content/uploads/2014/05/P30-Chassis-Manual.pdf

yes sir , i found that manual a couple years ago and it has really came in handy , and as far as the power steering fluid it was all changed a couple years ago when i replaced the steering gear box.

89 allegro said:

i will check that out and see ,most of the time im going straight and the brakes are not great ,but i will try turning it and see how that goes and see what happens.

Click to expand...

I meant to say do the brake and steering thing while the engine is idling and the rig is standing still.  The punp should be able to run both the brakes and steering.

When you replaced the power steering fluid did you bleed the line going to the Hydroboost?  You may have an air pocket inside the Hydroboost.

yep, i bleed everything that have a line or fitting ,then jacked up the front wheels off the ground and keep going back and forth for about a hour to get all the air out.

went out there today and turned sharp turns while on the brakes and sit still and turned them back and forth and it seems everything was normal .no jerking or anything ,so i guess the pump must be doing as it should ,so no reason to replace it as far as i can determine at this point.i guess the brakes are just as good as they can be for the age and size of this rig, so hopefully im done working on this thing for a while now and can get it ready to go striper fishing all this winter for as many trips as i can get in to westpoint lake and hopefully they dont close the campground down this year.thanks for all the help and advice from everyone.im sure there will be something else again at some point, just hopefully not to soon, thanks again everyone.

Sorry to reiterate but with the symptoms you describe (poor braking) I would still put the pressure test at the wheel cylinder on the to do list.

You won't know if brake pressure is correct until you measure it according to the manual...

The P30 service manual says normal brake line pressure is 1600-1850 psi.

it is impossible to get to my wheel cylinder lines because the are about a inch between the fitting and the leaf spring,i had to remove the backing plate to be able to change them last week because its impossible.can it be checked at the rear end where the rubber line T,s off and the lines go to the wheel cylinders? and where do you find this gauge? and i may give it a try just to be sure, if the pressure is low what does that mean ? is it hydroboost thats bad or power steering pump thats bad because when i get a chance i was thinking about just going ahead and replaceing it ,but i can get one from summit for over a month because i wanted ac delco and is all i can find is generic brands and cardone and i dont want to have yo replace it ever again so i want good parts.

You will probably have to buy and assemble the parts. One of my classic cars has a hydro-mechanical overdrive unit and I had to go to home depot to find fittings to adapt it to the port. I bought a 1,000 psi gauge from someone like McMaster Carr. There are, of course, higher pressure gauges.

You don't need to access the wheel cylinder, you need to access the bleed port, remove the bleed screw and replace with the adapted gauge assy.

As to what to do if the pressure is low, that's a good question. If the PS is fine then I would look to the Hydro Boost system.  The book says the pressure from the PS pump is like 1350-1450 psi and the pressure in the hydroboosted brake lines is 1600-1850.  This means the master cylinder must have a mechanical advantage created by the MC shuttle. i.e. a larger diameter piston on the input side vs. the output side. If I had low pressure at the wheels with good PS function I would be looking to replace the MC.